Haste is an effect found on spells, songs, and items. Haste, as the name implies, makes you swing faster; one
There are three types of haste that stack; item haste (which includes tribute,) spell haste (v2,) and certain songs/effects (v3.) In each category, only the highest haste will take effect, so if you have a 37% haste belt and a pair of 23% haste shoulders, you'll only get the benefits of 37% haste.
Anything listed in Lucy as v2 haste counts as a spell-based haste, and anything listed as v3 is "overhaste" -- it stacks with itemhaste and v2 haste.