Basic description of the class system
A class is the profession of your PC. There are eleven base classes from which you can choose at the creation of your character. These classes are: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Sorcerer, and Wizard. They all have their own abilities that sets them apart from the other classes.
Later in the career of your character you can choose to multiclass into another class if your character fits the requirements for that class, and if it has an in character reason. So, as an example: You shouldn't make a Monk/Druid/Shifter just for the AC bonus. You can also choose to level up in a prestige class, which are special classes that can be earned. Those usually expand upon an existing class. The prestige classes are: Arcane Archer, Assasin, Blackguard, Champion of Torm, Dwarven Defender, Harper Scout, Pale Master, Red Dragon Disciple, Shadowdancer, Shifter, and Weapon Master.
When your character reaches level 21 in a single class, or just in character levels, they become an Epic Character. They get access to superior feats called: Epic Feats. Wizards, Sorcerers, Clerics and Druids also get access to special Epic Spells. In an IC sense, when you reach the Epic portion of the game, it is expected that almost everybody knows or has heard of your character.