A character with more than one class is considered to be multiclassed or multiclassing.

While this allows a character to gain access to the features of many classes at the same time, it also risks a multiclass penalty.

Many class features rely on the number of class levels. Therefore, it's possible for a multiclassed character to dilute its potential by "spreading too thin" to be effective in all areas.

e.g.
a wizard 1 / rogue 5 multiclass can only cast the same spells (both in power and quantity) as a 1st level wizard, irrespective of the 5 rogue levels.

You start with your Base Class and then, somewhere on your next level, you decide to select a different class from your Base class; The character must still meet the requirements for that class (usually an alignment restriction for base classes, but more elaborate pre-requisites for prestige classes). The new class you select will start at Class level of 1.

• You do not skip the Class level of your initial Base class, when you continue on leveling your original Base class, nor the other Class you selected. Depending on your leveling choices, class level progression could look something like this:

Character / Race	C L A S S	Class Level
Level 1	your Original Base Class Level	1
Level 2	your Original Base Class Level	2
Level 3	2nd Class Level selected	1
Level 4	your Original Base Class Level	3
Level 5	3rd Class Level selected	1
Level 6	your Original Base Class Level	4
Level 7	3rd Class Level selected	2
Level 8	2nd Class Level selected	2
Level 9	2nd Class Level selected	3
Level 10	2nd Class Level selected	4
Level 11	3rd Class Level selected	3
Level 12	your Original Base Class Level	5

Again, you may also suffer Multiclass penalty for such decisions, although there are exceptions - typically based on race and favored class.