Pumps are needed when fluids or solids or gases need to be moved and gravity alone will not get the job done. There are two types. One is a reciprocal and the other rotary. An example of reciprocal is a bike pump. When you push down the air is pushed out of pump and into the tire. When you pull up up the air enters pump through a different hole. A rotary pump is like a water wheel in reverse with a power source.