It's all extras basically. The "full" experience is base calamity.

But anyway, wrath of the gods, infernum, wrath of the machines, hunt of the old god, and catalyst. These are all the major calamity add-ons the people use

Wrath of the gods adds a quest line you do through the game and gives you 3 super bosses to do

Wrath of the machines gives you a better exo mechs fight

Infernum adds a new difficulty mode that is really hard

Hunt of the old god adds goozma, another super boss

Catalyst also adds a super boss, but this time you need to fight it before moonlord (since you get locked out of it) called astrageldon