There are actually four basic classes: Cleric, thief, fighter, and mage. All of the others are modifications on these four, and there are a lot of them.

There are gazillions of source books that one can use, many different worlds to play in, lots of rules, regulations, optional add-ons, tables and charts to remove the need for originality and a brain, and an incredible collection of game modules to strip even the dungeon master, the only person in the whole arrangement commonly thought to require a brain, of the need.