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.
One of the basic Player Character types: barbarian, bard, cleric, druid, fighter, monk, paladin, ranger, rogue, sorcerer, wizard, or from the Epic levels. Class defines a character's predominant talents and general function within an adventuring party. It may also refer to an NPCs class or a prestige class.
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.
Epic
An Epic Class is different from standard class in that the game NWN defines the epic label as belonging to those character classes that have exceeded the normal 20 levels of D&D experience. Epic applies to those characters who have progressed beyond the initial 20 levels. Although the Epic levels continue to use the same experience point advancement formula, they may obtain superior skills known as Epic skill, superior feats known as Epic Feats, and other powers differently. When a PC has reached the epic levels, it became clear to the D&D game designers that the rules needed to change to balance the game dynamics.
Prestige Classes
Prestige Classes - New custom Avlis prestige classes.
Other Links
Classes (bioware) -- (NWN Wiki)