All pumps use basic forces of nature to move a liquid. As the moving pump part (impeller, vane, piston diaphragm,etc.) begins to move, air is pushed out of the way. The movement of air creates a partial vacuum (low pressure) which can be filled up by more air, or in the case of water pumps, water. This is similar to sucking on a straw. A partial vacuum is created in your mouth when you suck on the straw. The liquid is pushed up the straw because of the pressure differences between your mouth and the atmosphere.