Actually, the head is the height above the impeller at which the GPM is ZERO. The combination of the two hint at the torque of the motor driving the impeller. Since water is heavy, it takes force to push it up. The higher up, the more force needed since there is more potential energy in a higher column of water. (ooh...I am starting to sneeze from all the dust being kicked up...high school physics, 1987...aaaAAAACHOOOOOoooo!!)
Pond pumps are rated at different heads due to their use with waterfalls. A 3000GPM pump may only have a 500GPM @ 3ft head. Make sense? Since we typically aren't going up but more than 6-8", head ratings are not as critical as the GPH/GPM...as long as you have a nicely balanced flow channel. I hope it helps.