Common Drops: Zone-related drops and various common and fairly useful materials for both crafting and NPC trash selling, as well as helping you recover from battle.

Uncommon Drops: Monster-related drops, mostly common crafting materials related to the level range of the monster and area/planet you're on.

Rare Drops: Rare monster-related drops and uncommon crafting materials.

Very Rare Drops: Generally blue equipment, both armor and weapons which will vary based on the type of monster class you've defeated.