When you level up, you may add a new class to your character, which is known as multiclassing. When you add a new class, you gain hit points for that new class based on its hit dice and you add its base attack bonus and saving throw bonuses to your existing ones. Skills are gained as the new class with respect to class and cross-class skills and all new class features and feats are added as a member of the new class.

There is no limit to the number of classes a character may have, but the levels in all classes must be within one level of each other, or the character will suffer a 20 percent experience penalty. However, a racially favored class does not count against the character when determining if the 20 percent XP penalty applies. For example, a gnome that is 4th-level rogue/2nd-level bard does not receive a penalty because he ignores his bard class. But a gnome that is a 4th-level rogue/2nd-level bard/2nd-level fighter would receive the penalty, since his rogue class is two higher than his fighter class.

A multiclass character's level is the sum of all of his classes. So a 3rd-level rogue/4th-level wizard is a level 7 character. However, for arcane spell casting purpose, his caster level is still 4.