"It was not as if we'd stayed home and wasted our lives drinking wine with pretty girls."
- – A recurring motif in the Lay of Kraka
Vikings were Scandinavian people from the 8th to 11th century, a period in which societies based in Denmark, Norway and Sweden, making use of their long-ships set forth to trade and colonize areas including Northern France, the British Isles, Russia, Iceland, Greenland and even reached North America (though the settlements they set up there did not last). They also made a habit of bathing and washing their hands frequently, which at the time was unheard of among the peoples of Europe. Probably because they had to have about two dozen dudes on a small boat for a long time, so you would regularly bathe if you didn't want to be That Guy. They only stopped when France, of all countries, rolled a nat 20 on Diplomacy by offering Normandy to duke Rollo. One of his descendents by the name of William ended up with a claim to the throne of a place populated with Anglo-Saxons named Anglo-land (later known as England), and ultimately became its king.
Unlike popular belief, they did not wear horned helmets. This is for the practical reason that a big horned helmet might catch a sword unintentionally, which is all sorts of bad for the wearer; horned helmets were used on occasion, but only for ceremony. The ol' "horn-headed people eater" image was popularized during the 1800s.
As a final note "Viking" is not a noun, but a verb. Proper usage would be something like "Hay Olaf I'm bored and need some spending money want to go viking?" (The noun form would be víkingr, a person who goes viking).
Vikings believed that when they died in battle (preferably in a totally fuck-awesome way) they would go to a place called Valhalla to become one of the Einherjar (Chosen Slain) or to Fólkvangr (the realm controlled by Freyja, the Nordic goddess of love, prosperity, spring and being foxy as hell; also a death goddess and war goddess, which is why she gets half the chosen warriors in the first place), where they would chug booze, eat all the meat and cheese they wanted, and (if that actually managed to get dull) participate in massive murderfests only to be fully healed the next day and ready to do it all over again. On the other hand, if they died in bed or in a totally lame way (such as AIDs or cancer or... actually anywhere but battle is lame) they would instead go to a totally boring place called Hel where NOTHING FUCKING HAPPENED! EVER! (As you might imagine, this became problematic for many of their folk heroes who were just that fucking hard to kill). And if that weren't bad enough, people who committed what the vikings saw as the unforgivable sins, like oathbreaking, went to a prison overseen by the goddess of the dead. The ceiling is made from the bones of serpents, which drip burning venom, the halls are waist-deep in cold, slimy blood, and there is nothing to drink but goats piss and nothing to eat but rotten food (basically a Minnesota Vikings game). The exception is if you died while giving birth, then you got go to Valhalla; the vikings were surprisingly egalitarian in their attitudes towards the sexes.
That said, there was the third way to die. Dying at sea was totally cool for the Vikings, for while the Battle-junkies went to Valhalla and Freya, and the lame ones went to Hel, the Sea-Bears went to the Halls of Aegir, god of the sea, where they got their own Watery Valhalla.
Vikings aren't known for being nice, for a good reason. During their raiding parties, after killing, enslaving and/or raping the non-Viking people they encountered, the Vikings would they would steal anything that wasn't nailed down. If it was nailed down, they'd try and steal the nails and if that didn't work, they'd eat it. If they couldn't eat it, they'd burn it, and if they couldn't burn it they'd 'SMASH' it!
Later some of the smarter Viking warlords started to conquer shit rather than rape, pillage and kill everything in their sight. For quite a long time a large chunk of France and Italy, and the entirety of England and Russia were ruled by Vikings or their descendants, although they all got quickly assimilated into the nations they've conquered, to the point when they started to think of themselves as French/Russians in just a two or three generations after settling in. The Vikings also had a level of prestige in the Byzantine Empire, as they were the preferred recruits for the Emperor's bodyguard, the Varangian Guard.
Like Greek mythology, the Norse have their own version of creation, different sets of gods, and heroic stories of manly feats. Here are some of them (Note that, much like Celtic mythology, Norse mythology was only written down long after Scandanavia had become Christian, so there remains a massive amount of missing stories (for example, while the war between the Vanir and the Aesir is mentioned, we don't actually have the full description of it, even though at one time it probably existed).
In Modern Fiction
Vikings and the honorable Neanderthals are some of the closest that the real world has ever had to dwarves, but they should not be confused as such. While they had a penchant for axes and could use anything, including body parts and broken furniture, as a weapon, Vikings were just unspeakably awesome humans (they couldn't handle as much booze as a dwarf, though only just). Vikings that rode Dragons even more so. Vikings are not to be confused with barbarians either, despite any combination with the former resulting in awesome. Vikings are also notable for pledging themselves to Chaos and becoming werewolf supersoldiers.
The Vikings have also finally gotten their own TV show starring Vladimir Kullich. It is about the saga of Ragnar Lothbrok and his sons; Bjorn Ironside, Ivar the Boneless, Sigurd Snake-eye, Halfdan, Hvitserk, and Ubbe, as well as the tales of Duke Rollo of Normandy, King Harald Fairhair, and Alfred the Great of Wessex.
The thing that put the Vikings on the map were their Longships (or LongBOAT if you're not American). Basically these were large canoes made from planks with a mast to catch the wind. They could, however handle rough northern seas very well, and allowed some Vikings to reach such exotic locales as Newfoundland centuries before other Europeans. One thing that helped made the Longships such a gamechanger was that the vikings worked out that properly curing and drying out timbers it made it stronger and more resistant to being eaten at sea by nematodes and similar grody things.
Sometimes to save travel time, the Vikings would pull their Longships overland for kilometers. No joking, no hyperbole. A few tricks (like log rollers) helped, though. One of them (Oleg, the prince of Kievan Russ) even mounted his longships on wheels to quickly move them into Constantinople harbor, bypassing the defensive chain pulled across the path (which possibly inspired the Ottoman sultan Mehmed II when he used a similar trick to help him capture Constantinople).
Their Longships also had an early warning system so that people could tell wether they were going to fuck them up or not. It's to do with the shields: If the Shields were on the outside of their Longships, then they were coming to trade goods. If the Shields were not on the outside of their Longships, then they were going to use them in battle, and you should run for the hills (if you get that far...)
There's lot of bullshit about these guys on the internet and in general beliefs. Hell, the word itself had became the synonym of uncontrollable rage in many languages. The truth, however, is quite boring - berserkers (which comes from the Old Norse for "bear hide", as it was their signature piece of clothes they wore above armor, or sometimes instead of it) were equivalents of champions in the Norse culture with a pitch of warrior-priest flavor added - i.e. the guys who fought in duels on behalf of the tribe or some wealthy noble. And Norse culture had a fuckton of things settled with duels. As best of the best professional warriors among already brutally strong vikings they kicked all kinds of asses, and were rightfully feared for their skill and bravery. As you may guess, they where quite rare, so no "hordes" or even "squads" of berserkers for you - at best you'd have two or three per raid, and most often only one. As for uncontrollable rage... well, sagas mention a total of ZERO berserkers going into what we now call "berserker rage" - there are mentions of jarls and ordinary warriors going to battle biting shields, foaming with mad anger and killing friend and foe alike, but never berserkers. WRONG:
"-And as the foemen's ships drew near,
The dreadful din you well might hear
Savage berserks roaring mad,
And champions fierce in wolf-skins clad,
Howling like wolves; and clanking jar."
- – Harald Fairhair Saga ch 19.
Mushroom brew painkiller that allow to fight despite heavy or even fatal wounds likewise weren't their exclusive, although proper brew (that wouldn't ruin your liver, therefore sentencing you to a lame death in your bed if you survive the battle) was quite expensive, and berserkers, as pretty much second-in-command of jarls were among those wealthy enough to afford it.
When Elspeth Tirel needs backup, these are the people she calls.