Multiclass

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

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

Many class features rely on the amount of class levels, so there's a possibility that a multiclassed character might be spread too thinly to be effective in all areas.

e.g.

A wizard(1)/rogue(5) multiclass will only be able to cast the same spells (both in power and quantity) as a 1st level wizard, irrelevant of its 5 rogue levels.

Multiclassing
A character becomes multiclassed by taking a different class from their current class at level-up.

The character must still meet the requirements for that class (usually an alignment restriction for base classes, but more elaborate pre-requisites for a prestige class)