Pumps are machines that transfer fluids by mechanically adding energy to them. There are two main types: centrifugal and positive displacement. Centrifugal pumps use centrifugal force to push liquid outwards from the center, while positive displacement pumps have mechanisms like lobes, gears or diaphragms that push discrete volumes of liquid out of the pump. Centrifugal pumps are generally used for high flow, low pressure applications while positive displacement pumps are used for low flow, high pressure applications or high viscosity liquids. Major components of centrifugal pumps include an impeller, casing, shaft and seals, while positive displacement pumps have components like lobes or gears that expand and contract pump chambers.