Classes
A Class is the profession or vocation of your character. It determines what he or she is able to do: combat training, magical ability, skills, and more. In selecting what class is right for your character, keep in mind what base strengths you want him or her to have.
Multiclassing
PCs begin with one base class but, as they gain experience, may choose to take a second or third class (base or prestige) -- which can result in a multiclass penalty if certain requirements aren't fulfilled. Characters may have a maximum of three classes.
Prestige Classes
Prestige Classes - New custom Avlis prestige classes.
Other Links
Classes (bioware) -- (NWN Wiki)