It all depends how long and how extreme you go with the pumping. You can pump to the point of body modification.
But in general, think of it as a hickey. When you suck, it brings the blood to the surface of the skin which causes the discoloration. So when you pump, all it does it engorge the area with blood, causing swelling.
As far as a penis pump goes, the effects are temporary (unless as I said above you are extreme with it). As soon as the suction is removed the effects begin to wear off.