Your class levels are a representation of how experienced your character is in a particular category of skills. You gain a class level every time you gain a character level, and you may level a class tree you have already begun, or start a new one at level 1. This is known as multiclassing. If you have multiclassed 3 Barbarian levels and a 4 Wizard levels, you would be Character Level 7, Barbarian 3: Wizard 4.

CL7-Barb:3-Wiz:4.

above: An abbreviation of the example multiclassed character’s Class Levels

Some class resources have an amount of uses based upon the class that granted it’s level.