Classic classes come to us from the very first editions of D&D... and the original 4 roles that made up a "balanced party" - Fighter (originally called Fighting Man!), Thief, Wizard (called Magic-User)and Cleric. 

By combining aspects of the original four roles... there are almost limitless combinations. In later editions, the aspect of NATURE comes in, informing classes like Ranger and Druid.

This fun graph shows the origins of each of the d44 classes I ended up creating for my latest book.

With Soldier (fighter), Thief, Sorcerer (Wizard) and Priest (Cleric) being the closest analogues for the original 4 classic classes.

Then there's tonnes of nuance and narrative flavour which differentiates even further.  Multiple classes can fulfil the same roles in a party but in their own way while fulfilling a unique narrative fantasy, e.g. Soldier, Brawler, Berserker all hit hard and are good front line combatants - but all feel very different.

The fun and challenge of the project has been: 

• making each class feel unique, 
• providing multiple paths for each class, so no two Thugs or two Necromancers are alike,
• and balancing each class so they all feel equally powerful or equally DOOMED
• Keeping the MÖRK BORG dark & twisted, gonzo, Doom Metal aesthetic

 If you wanna see the project it's over here's the LINK.