I am guessing + damage (added damage) is just added on the end of the damage number on a spell cast for the different spell damages and melee/bow attack for physical? Then +% damage (enhanced damage) increases the total number by that % after the damage is calculated for the total amount of damage done?

I am pretty sure that is how this works, just making sure I understand it correctly.

I assumed that +% damage only applied to that damage type. So if you had +100 poison damage on your gear and +30% poison damage then you'd be doing 130 poison damage. I'm curious how it works as well, that would probably change a lot of my gear choices if it were +30% of your total damage added as poison!

The + damage stuff only applies to melee attacks. It's the additional pop-up damage you see when you melee. Casters want stuff that says +% damage because that applies to everything. Even druids benefit from + physical% on tornado and earthquake.

How does +physical work for melee? And %physical?

Almost nothing special about them particularly. They're handled just like their elemental equivalents. The only difference is that you can potentially leech from +physical damage, so that may be a consideration.

So just to clarify, +% elemental damage only works on spells that use that element or the "+____ elemental damage" from your gear? Or does it add +% of your TOTAL damage to every attack as whatever element?