Psychic Vampires draw on the energy lifeforce of a being, while Bloodsuckers, as their name implies, merely draws on the physical lifeforce of the being.

Psychic Vampires generally get away with feeding a lot more than their hemophiliac counterparts, since there's no laws against attacking a person's energy and generally no way of knowing that they fed off of you, save that after just being in their company you will usually feel very exhausted.

Of course, we can also feed off of anything that holds energy - computers, batteries, wind, energy drinks (for us they quite literally live up to their name) etc...anything that has energy can be drawn from.