A class, as roguelike games borrow the term from role-playing games defines certain common characteristics among a group of conceivable characters. These include tendencies in their attributes, the ability to perform certain actions, and the ability to use or wear specific kinds of equipment amongst many other things.
Some roguelikes use classes derived directly from Dungeons and Dragons, such as
Warriors tend to be physical based, attacking their enemies straight on with weapons rather than through magic or trickery. Warriors are generally best when wearing heavy armor and/or heavy weapons rather than light armor or sword and shield.
Rogues are a very speedy class, rogues tend to be adept in lock-picking and trap detection skills in games which have them. They usually use daggers and light armor if they get in fights.
Mage (Magic-User; Wizard)
Mages usually prefer to fight with magic rather than swordplay. Mages have lots of versatility with their spells and abilities they gain from their studies. Some games might opt for them to use wands with limited charges rather than casting from memory, they often wear robes.
People who worship a god with much more devotion than other people. Their abilities usually make staying on their god's good graces much easier than other classes. They often use blunt weapons and wear robes.
Paladins are champions of law and order. In many games they are perceived as a much more defensive warrior, they are often clad in heavy armor and wield swords and shields.
People who study the martial prowess of their own body, a good monk generally never needs to wield a weapon and lets their fists do the talking. They are often wearing light armor or robes.