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, so there's a possibility that a multiclassed character could be spread too thin to be effective in all areas.

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 that you selected will be starting at Level 1 Class level.

Character / RaceC L A S SClass Level
Level 1 your Original Base Class Level1
Level 2 your Original Base Class Level2
Level 3 2nd Class Level selected1
Level 4 your Original Base Class Level3
Level 5 3rd Class Level selected1
Level 6 your Original Base Class Level4
Level 7 3rd Class Level selected2
Level 8 2nd Class Level selected2
Level 9 2nd Class Level selected3
Level 10 2nd Class Level selected4
Level 11 3rd Class Level selected3
Level 12 your Original Base Class Level5

Well, You get the Idea.

You will also suffer Multiclass penalty for such decisions, but there are Exceptions.