Money is a monetary token used for trade. Bartering using resources was typically not always a reliable form of commerce, as value of goods typically varies from place to place; one area may favor the goods you're trying to trade with, while another area might consider it worthless. A centralized form of payment was often established in place of it so that people would have an easier time in gauging the value of a product and trying to pay for it. Lots of things have been used as money from fancy shells to grain to plugs of tobacco to valuable lumps of metal to notes backed by the authority of government. MOST IMPORTANTLY, money allows YOU to buy cool toys so that you can become part of /tg/ while keeping another money-whoring company alive. Show them your love of currency by paying for your next box of Space Marines with a Yap Stone.
That said, money only works if the place you're trying to spend it at has use for it (after all, all the money in the world is useless if you can't get anything with it). A town with a well-connected trading post will welcome your coins with open palms, while an isolated village in the middle of nowhere would find those hunks of metal to be worthless and would prefer you give them something actually useful in exchange instead.
A Brief History of Money
The concept of trade predates civilization, but civilization increased the need for a commodity which could store value and be easily exchanged. Salt was one of the earliest commodities humans used for trading with, as (in ancient civilizations) it was universally in demand, could be divided easily, and didn't spoil (though it needed to be kept dry), problems which plagued other commodities such as livestock, grain, or metals.
Eventually by trial and error humans discovered that some metals are rarer than others. Gold and silver in particular were attractive as they could both be worked easily, did not rust (in the conventional sense anyway), and appeared to exist in a fixed ratio to each other (17.5 silver to 1 gold on Earth, close-ish to the theoretical ratio from stellar sources). This made them good to use as a token that could be universally exchanged, but gold and silver are still heavy to lug around for transactions, while being all too easy to carry away by robbers.
So what people settled on was trading their stuff for precious metals, and then storing those precious metals in a bank and trading notes around that represented precious metals. Of course, this had some teething problems, the earliest banks (and some recent ones) weren't very trustworthy. Eventually governments got a bit greedy themselves and decided that pinning currency values to metals was limiting their ability to spend irresponsibly, and so they removed that connection and that's where we stand today. Money has value basically because people believe it has value, and as long as that's the case we're okay.
When it's not the case, the result is Venezuela, where the Bolivar is worth less than WoW gold.
Money and /tg/
Being a tabletop hobbyist, money plays a huge role in the life of every neck beard. If you don't have money you can't build your army, without your army you cannot participate in tabletop match. The require products for this hobby comes with miniatures, paint and codex. They are all damn expensive, especially when they are all measured in euro (which is currently the most expensive currency of all).
Suffice to say, wargaming/miniature modelling is a first world middle class/upper middle class hobby, but mostly for younger audience until a certain age above 25? because once a person is past a certain age threshold, they'll be too busy at work than spending time on their hobby, or not because they could just become a lazy manchild who doesn't work but use the warefare or their parent's money like the fucking parasite they are. A recent move was made by GW to attract younger audience into the bloody world of 40k, with improvements such as making newer edition less complicated, advertising their games using smurf-sues or introduce the grimdark world of 40k to a much younger audience. This turns out to be a great moves, as it turns the children into plastic crackhead, forever bothering their parent to buy them a new box of space marines. Some hobbyist with little income prefers to buy their miniatures one by one when they have the money to spare, until they can finally compete with other player......only to bitch online when their favorite faction lose to a bunch of blue face space communist.
Piracy is often seen as an alternative to currency transactions for obtaining RPG products. Unfortunately this can normally only obtain digital goods, though with a 3D printer miniatures can now be pirated as well. This is illegal, but rarely enforced past the provider if you're not stealing movies from a major distributor or something written by a total sperg like Zweihander. Another alternative is the use of simulators like Tabletop Simulator or one of the many card game simulators, which can allow copyright infringing representations of physical objects to be used.
Money in games
Money is a common feature in many games. It's an idea you would naturally be familiar with, is a natural part of almost any complex agricultural society not designed by the dirtiest of hippies and is for providing incentive to players in story creation. On top of that is easy to express with a pen and paper. Your Paladin has acquired 50 Golden-Wombats so add fifty Golden-Wombats to their inventory, you subtract 30 Golden-Wombats from your inventory when you buy a new shield from a blacksmith and so forth.
In settings where the system of currency has collapsed or is non-existent, like a post-apocalyptic world, money is often eliminated, forcing people to stick to bartering. Alternatively, a new "currency" based on a useful item develops, such as Fallout system where trade is conducted using the water standard (the Water Merchants will trade a fixed volume of clean water for a cap) backed bottlecaps, while Metro 2033 universe has people trading in pre-war ammunition after Moscow's economy was obliterated in the setting's nuclear war. Gold, merely being pretty and useful for making electronics that can't be made anymore, rarely retains much value after the apocalypse (Silver, which is useful for its anti-microrobial properties and making mirrors, would actually be more useful, but this is rarely touched upon).
In setting like Path of Exiles, where the player is but an exile, rejected unto the cruel lawless continent of Wraeclast. The inhabitants there has an unique currency system where they trade useful magical orbs that can changes the properties of the weapons and armors. Each orbs has their own value based on their ability to either change or improve the properties of the gears, and this one orb "that can randomize a rare item's property" is worth to 10 of that orb, whose ability is to "randomize the property of a magical items" (similar to 1000$ > 100$ > 10$)
Money in Fantasy settings
The default type of money in you usual fantasy world are coins following a simplified version of the Roman System with (in descending order of value) gold, silver and copper pieces, sometimes with platinum on top of gold (even though it wasn't worth much till more modern times) and electrum between gold and silver (even though the percentage of each metal would be impossible to determine and only hold more value than silver from a state's backing and assurance of a particular degree of purity). Such systems are usually made by people who are either lazy or don't hold the nature of local currency to be of much concern of the worlds of the story that they're writing. Those that go more in depth or want to differentiate different cultures and civilizations will add different types of money. Those who want to simplify things even further simply stick with gold. Gold's density is usually left unaddressed; a typical "fantasy coin" (1" diameter, 1/8" thick -- gold is too soft to be made into thin coins) of pure gold would weigh a little over an ounce (31 grams to be precise), so a sack of 100 coins (enough to buy a decent sword or an adventure's worth of supplies) would weigh nearly seven pounds, and never mind lugging around 1000 gold coins (68 pounds) to buy a suit of armor.
Another challenge of gold specifically is that one of the main goals of alchemists was working out a way to transmute common metals into gold. In a few fantasy worlds there are various types of magic that can do this. Such magics would inevitably lead to inflation. This might explain why gold coins are used as a day to day currency in many fantasy worlds when in real life it was unusual for people who were not merchants, nobles, high end artisans or scribes to deal in gold rather than in silver or copper.
Most common among the more exotic options for currency in fantasy settings is souls. Trading in souls is an extremely popular trait for any evil society with good grasp of magic, especially extraplanar beings. More rarely valuable items, that are for some reason valued the same everywhere, like art or food can be used as a money equivalent. Most adventurers turns this kinda thing into cash the first chance they get.
For some reason dragons also like to sit in piles of gold and gems, which makes them targets for adventurers trying to seek wealth.
Wouldn't it be great if coins were made of harder, less dense metals so they could be thinner and lighter? Or heck, we could just dispense with the metal and write down how much we're worth or spending on pieces of something really lightweight, like paper. Nah, this is crazy talk!
Money in Sci-fi settings
In futuristic settings, money is usually just a single unit (typically called "credits") and largely operate on a digital level instead of people having to give a form of bank note or coin. In most sci-fi settings that include aliens with their own currency system, the proverbial credit is usually the standardized form of money people pay with that has a direct equivalent to their home system. Basically similar to what the European Union did with the Euro, but on a larger scale and somehow managing to go decades without a member state's financial issues fucking it up.
In a few science fiction societies such as the Federation from Star Trek and The Culture have done away with money, with economies which are so efficient that it is easy to provide fulfill material needs due to robots, replicators, fabrication machines and similar there is not really a reason to pay people to work so society just provides people with what they want/need for free with all work being voluntary. The problem of such a "post-scarcity" economy is that creativity (without dangerously intelligent AI), land/space, entertainment, sexual gratification (without sexbots or the like), and attention will still be in finite supply. In less idea situations often posited society gets dominated by megacorportations in which the working masses toil for scrip, coupons which can be redeemed at company stores for company products. This sort of thing did happen in the 19th century a lot as a way to keep the workers in line in company towns.
Money in 40k
In WH40K; currency is not completely detailed upon. The Imperium officially runs on a feudal system with no unified currency system, where each world must pay tithes of various resources such as food, minerals, hardware, manpower.....etc, instead of paying in coin. Although several publications say that each Imperial world has their own local currency system, not to mention that guardsmen also get paid with actual money (although unspecified). Most people typically just accept that Imperial currency is either called "credits" or "thrones" (throne gelt, specifically, and yes that does indeed mean they are Golden Thrones), since they're the most prevalent.
Additionally, every human of the Imperium is indebted to the Emperor at birth, as the only reason they're alive is because of the Emperor's continued sacrifice upon the Golden Throne. Thus, every citizen of the Imperium is considered a currency of the Emperor, and can be spent by the billions to preserve it, under the hopefully competent command of one of his generals. That said, wasting something that belongs to the Emperor himself is heresy, so while a commander or an Inquisitor may have the authority to spend that much if needed, they better have a good reason for it (those billions of guardsmen you sent to die on a pointless objective that did no favors to the Imperium? They could have instead been used in active, high-priority warzones like Cadia or Armageddon. You have potentially cost the fate of the entire Imperium through your incompetent squandering of His faithful servants, execution for such a grave offense.)
For Orks, they use teef, which can be obtained by krumping a git and yanking his chompers out. To orks; bigga teef means bigga value, so Orks who wants the killiest gubbins from the meks typically has to pick on someone his own size, or someone larger. Orks also run on a bartering system, where one can trade something of value; like salvage or resources to get what they want. Because teeth rots overtime, it cannot be hoarded for long, and has to be spent constantly to get the most out of it. And because no Ork can get rich by just sitting on his ass, this helps keep an Ork band eternally thriving as the constant fighting helps stimulate the Orks' growth. Some Ork bands also do not care if the teeth isn't from an Ork, so long as it looks like teeth and is zoggin 'uge. Its for this reason the Tyranids are another popular source of teef.
Money in Star Wars
In Star Wars the Republic Credit was the standard currency Galaxy wide, backed by the InterGalactic Banking Clan. Credits primarily exist electronically but also exist as untraceable, secure chips with a number of credits on them and (very rarely) metal coins. The Credit weakened toward the end of the Republic and many alternate local currencies popped up, most notable the Hutt Peggat, which was coined in gold and held a value at 40 times the credit and came to dominate Outer Rim transactions. When the Clone Wars broke out the Credit hit its lowest point as the Confederacy began printing currency that was broadly accepted, but this Confederate Credit died with its printer. The Empire replaced the Republic Credit with the Imperial Credit and ended the decades of low faith in the Credit. The Alliance to Restore the Republic issued its own credit, but it held little (if any value) for outside transactions and they primarily used the Imperial Credit till they became the New Republic. The ensuing Balkanizing of the Galaxy as the New Republic began to officially control worlds and the remaining Imperials broke into a maze of factions created another mess of currencies. Since no two governments considered the other legitimate, traders had to rely on commodities to make transactions. This lasted till the Pellaeon–Gavrisom Treaty brought peace between the New Republic and Imperial Remnant.
Money in (relatively) Modern Settings
While barter is typically associated with extremely primitive societies, it has seen use far beyond what people realize. In the Thirteen Colonies, hard currency was in short supply due to limited minting and a tendency to leave across the pacific and not come back. Heavy regulation on producing currency (one of the many, many causes of the Revolution) forced the colonists used barter, trade goods, "I owe you"s, and foreign coins of subjective exchange rates for trade and wages, a system that was reinforced by barter with Indians. Barter in the colonies was so developed that Matthew Patten's diary (a major primary source for the period) was largely spent keeping track of what he owes people and what people owed him.
Money in Even More Modern Settings
In theory, representing the currency system of an Industrial Revolution or later setting should be easy, as it's essentially a system everyone already knows. In practice, real monetary systems are far too volatile to be realistically represented in a game, as values can vary heavily not only year to year, but month to month and place to place. Further complicating things is that real money costs aren't really aligned with the gameplay needs of player characters or balance, and a majority of income is typically spent on living expenses that don't impact the game at all. As such, several systems have created systems to avoid dealing with this.
D20 Modern introduced an abstract wealth score that allowed ignoring costs for minor purchases and not keeping track of modern fiances. This system is a total clusterfuck. Among its flaws are the high degree of randomness, which in turn requires the DM be present for character generation pre-errata (post-errata you can take 10 there to avoid it, though large purchases still have random costs), odd values, and generally being more unwieldy than the basic addition/subtraction it replaced. d20 Apocalypse introduced a barter system, but it was non-comprehensive, geared toward a very specific setting, and largely dart board in its value.
The original, White Wolf 3.5 version, and fan-made DM's Guild incarnations of Masque of the Red Death all use relatively standard for the medium depictions of wealth. The RPGA variant however, takes the interesting approach of giving characters an amount of money between adventures and declaring it represented merely their disposable income, explicitly rejecting the idea of them being pennyless hobos and leaving their personal finances to behind the scenes. Another interesting thing done by this version of the game is it explicitly encouraged use of period mail order catalogs as a supplement to its meager equipment list. Sadly, this came before they were all digitized and easily obtained as free, public domain, PDFs on the internet and required reproductions or the distant hope of finding one in a library. Made worse by the RPGA requiring players bring all used sources, so its suggestion of checking the library is instantly rendered non-viable.
Some games have ditched use of any form of currency to buy effects entirely, and instead price things purely on their mechanical effect. d20 Modern's child Spycraft 2.0 has player characters get access to new and more powerful gadgets as they level up. This system is abstract and can represent issued gear, stuff the character created, or just what they can spare for their adventures. Mutants & Masterminds has equipment (generic mundane items) and gadgets (unique, special, items) priced in character points based purely on what gameplay effect they generate: Items that produce Ranged Damage 4 costs a fixed number of points, regardless of if it's a space blaster or magic bracers that let you throw lightning bolts. A character can be wealthy for a low cost, reflecting that while it may occasionally enable creative solutions (buy out the only company that can supply an important component the bad guy needs) it's not going to make Godzilla go away.