The base classes are Warrior, Mage, Rouge, Cleric, and Hacker. You may select your class only once and I mean once. You will not be able to change your class later, so check every class feature and decide which base class you want to be the most. Don't worry, you can level up your classes once you reach a certain amount of posts, so you won't have to use the same class name over and over again. However, the maximum level for a class is the level at 500 posts. Classes affect your abilities and advantages in battles, as well as your type of damage and more. Just in case you're wondering, you may not alternate any class names or abilities for any reason. What's there must be followed or no class at all... which would make your character more vulnerable. Once you have became a certain class, you can join their Usergroup. Ex. If I was a Mage, I could join the Order of Mages.
P.S. Class abilities will consume your AP bar. Each ability will cost one *, which stands for ten Ability Points. Also, when you upgrade your class, you get to keep the abilities from the previous class.
Note: Even though you might not realize it, all classes are the same in power levels in some way. Even though some classes may seem over-powered and others, pathetic in power, they're all good and bad in some way, so please don't complain (too much, if not at all) about it.