In his series entitled "The Fundamentals of Pumped Water Systems," Robert Pelikan starts with a look at the heart of any pumped water system: the pump.
Water, and other fluids for that matter, can be moved in many ways. If water is above where you want it, gravity can get it to its destination. If you need to get it higher than where it is, you'll need a water wheel or some sort of pump. There are ram pumps, piston pumps, gear pumps, diaphragm pumps, lift pumps, centrifugal pumps, etc., each having its own place in the sun (or under the ground as the case might be). Some are positive displacement pumps capable of moving relatively small amounts of fluids at very high pressures, way more than is needed in most water system applications. Others, like the water wheel and centrifugal pump, don't produce much pressure or lift, but can move very large amounts of water. But, if configured properly, as will be pointed out later, centrifugal pumps can produce a respectable amount of pressure.