Most characters and creatures have levels in more than one class - usually at least one class in each of the mind, body and experience categories. The class abilities from a character's different classes combine to determine overall abilities.
Often (almost entirely, in initial playtesting), the abilities, characteristic values and bonuses granted by different classes are separate from each other. Where more than one class grants the same kind of bonus or characteristic increase, as a general rule they stack: add together the numbers to give the total bonus or characteristic value.
"Character level" (alternatively "total level" or "overall level") is a character's total number of levels.
"Class level" is the number of levels a character has in the particular class in question. Where context does not make it clear which class is being referred to, it should be specified, eg "Fighter level".
A multiclass character gets all the class features of all his or her classes but must also suffer the consequences of the special restrictions (if any) of all his or her classes.
Characters gain feat slots by class level (not total level - different classes grant feats at different rates), and each feat must be chosen from the list of the class that granted the slot. Characters gain the benefit of all feats they have and meet the prerequisites for, regardless of which class granted the slot.