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.