A class is, in the broadest sense a division or category of similar things. A class of ships all have similar designs and roles, a class of students may share an age, achievement level, or graduation date, and so on. In the context of role-playing games, there are two definitions of class that are most relevant: character class, which is a career or set of skills, and social class, which is a character's place in the social hierarchy.
A character's class (e.g. ranger, paladin, rogue) encodes and/or influences her career (in fact, some role-playing games use career as the term for the class mechanic), skills, specializations, and role in the party. A character's class may provide bonuses to certain stats and grant access to certain skills (or make certain skills cheaper to purchase) as a character gains levels in that class. Some games permit multiclassing, where a character can pick up levels in different classes at the same time (leading to character classes like "Paladin 6/Wizard 1/Lawful stupid 10" and the like).
In Munchkin, you have no class (heh) until you play a class card.
As most fantasy games (and some sci-fi games, like Warhammer 40,000) are set in a quasi-feudal time period, their societies tend to be fairly stratified. Feudalism and class systems from history could get quite complicated, but most game writers (and most players, for that matter) abstract things:
- Royalty is usually at the top of the chain. There might be some layering if the kingdom is a vassal state of an empire.
- Below them are the nobles. Often, there's too much land for a single royal family to control personally, so they parcel it out to families loyal to theirs, who might in turn divide their chunk into smaller pieces for lesser noble families to manage, and so on. Nobility is broken up into various stratified subclasses from those who rule provinces bigger than most kingdoms to podunk knights who are in charge of a square kilometer of marginal rocky farmland and a dozen dirt farmers.
- Some powerful organizations, like a state church or a society of wizards, may sit parallel to the nobility -- they aren't families and don't have inherited titles, but they have holdings of comparable size and some kind of royal recognition.
- Artisans, guilds, and similar organizations occupy the next layer down. Their members are skilled and respected, but they're not born into noble families, so they don't get to have castles or armies. As roads improve, ship designs get better and cities grow these classes get more powerful. Most player characters are (or at least start out) in this layer.
- The largest group are peasants (or "commoners" for a more polite term) who grow the food that everyone eats. Not all of them are happy in this situation (cf: the French Revolution).
- The lowest class is the underclass. Pretty small in the grand scheme of things these guys do the really shitty jobs such as being nightsoil men and scavengers who looked through garbage middens for anything they might scavenge. If you're in this class everyone looks down on you.
- (Special mention should go here to the "classes" of slaves and serfs. They don't really merit a formal entry as such on this list; Any society that practices the institution will almost never consider the slave population to be "human," or whatever the predominant species is, much less worthy of classification as anything other than chattel.)
Usually, the only way to move up a layer is to marry someone from a higher layer (good luck to those peasants in love with princes and princesses), get elevated by members of that layer (you might get a knighthood, but your kids won't inherit it), or force your way in by revolution (in which case, watch your back for people looking to do the same to you).
If one has an interest in East Asian societies and wants to use that as a template, the social hierarchy is structure a bit different due to Confucianism. It's laid out roughly as such...
- Scholars: Scholars are rich people (generally landowners) who have money to dedicate their lives to study of moral philosophy, history, arts and similar. This is a big deal because you get government jobs by passing exams about these things. Their wisdom and learning is to be valued above all. Fortunately for people who are not born into this class you to can become a scholar if you can pass the exams. Though you are going up against people who have been training for them from childhood and about 1 in 500 or so gets a passing grade. But there is paper evidence of it happening so it's a better chance then you might think of a peasant getting into the top of society. In Japan, you substitute Samurai for Scholars.
- Peasants: Peasants are technically ranked pretty high because they keep everybody fed and clothed. Without them, everyone would starve to death. The cynic would argue that this "high rank" is just a ploy to keep peasants happy and avoid rebellion.
- Artisans: Artisans are seen as less critical than peasants (if you don't have shoemakers you have to make your own shoes or go barefoot where as if you don't have peasants you starve to death) but still valued for their skills and abilities in taking what the peasants produce in terms of ore, clay, wood and food and making nice and important things out of them.
- Merchants: Merchants are at the bottom of this hierarchy. The peddlers are USEFUL, but they don't create anything. All they do is buy stuff from one person, move it, and sell it to someone else. In practice the merchants gain prominence and influence above their station by virtue of their wealth, much to the annoyance of the Scholar class and generally in spite of their best efforts to keep them down.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
|Dungeons & Dragons 5th Edition Classes|
| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
|Artificer - Mystic|
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|