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:

• D&D set the standard and follow them to this day
• Most Storyteller system games had a softer version of this
• Apocalypse World has playbooks which are towards the harder spectrum
• Tenra Bansho Zero and many Japanese RPGs have class-based systems

As a system feature, Classes are not inherently good or bad.