1. Impeller type works just like the raw water impeller on your boat motor's cooling system. A splined impeller runs in an eccentric circle causing it to suck water on the intake side and push water on the discharge side.
2. Aerator type spins water, centrifugal forces develop pushing water out of the outer housing and pulls the water through the center. Air doesn't weigh enough to make centrifugal forces high enough to keep it primed so it has to be flooded to begin the pumping process.