A system feature where PCs have access to skills and unique or rare abilities based on a concept, introduced in D&D: Fighter, Thief, Wizard.
One reason for this is to make sure everyone in a group bring something to the a group of PCs (role protection), another to speed up character creation.
A “harder” version of this severely restricts the character mechanically while a “softer” allows greater flexibility with what can be done within a class.
Multiclassing is when PCs can combine the mechanics of multiple classes, usually at a mechanical penalty.
When done badly, some classes will be overpowered while others will be extremely weak (Wizard or any “Caster” class versus Fighter or “Martial” classes in D&D 3.x).
Some games with class mechanics out of many:
As a system feature, Classes are not inherently good or bad.