Air compressors are devices that use motors to force air into a storage tank, creating a pressurized container of air which can be used to power tools, force air into other containers, and more. Modern air compressor motors are engineered to stop adding air when the container reaches a certain psi (pounds per square inch). They also are designed to continue adding air when the psi is too low.
The two considerations when purchasing an air compressor are where you will use the compressor and for what purpose. These two questions will determine whether you need a portable or stationary model as well as at what horsepower and capacity.