Reciprocating (Piston) Compressors can be widely found in two primary configurations;
Single Stage and Two Stage. Single stage air compressors work by drawing air in and subsequently compressing the air to its final pressure in single piston stroke. Single stage air compressors can attain pressures of up to 150 PSI. Typically, a single stage pump will have a higher CFM (Cubic Feet per Minute) rating than a two stage pump because every cylinder is drawing in air and compressing it with air during every rotation.
Two stage air compressors work in a very similar manner with the primary difference being that they compress the air in 2 steps or stages. During the first step or stage, air is drawn in and compressed to an intermediate pressure. After being compressed in the first stage, the air is piped, usually through an intercooler where the air is allowed to cool, to be compressed in the final or second stage. Two stage compressors are normally good for pressures up to 200psi. Two stage pumps are more efficient at higher pressures because the air is cooled between the stages.