There are many different types of air compressors. They vary in both size and the power they supply. Obviously the best way to determine what air compressor to use is based on the power requirements of the tools involved. The next step is to buy one that gives you a little more power than you need so you can purchase air tools in the future that may need a higher power level.
One common example is the rotary screw compressor. There are many different choices with this product line, but if you want a powerful one, you will have to spend some money, the cheaper ones are only good for putting air in your tires once in a while.
Don't rate air compressors by Horsepower, as often the hp listed is an inflated number. Instead, look at the PSI, which stands for Pounds per Square Inch. In most cases, you will require 90 PSI for most air tools. However, an air compressor listed at 125 to 135 PSI will not run that air tool properly. This is because the PSI is the shut-off pressure, and even if an air compressor shuts off at 100 PSI, this does not take into account something called pressure loss in the line. Thus, to work at peak efficiency with 90 PSI tools, you should really consider a two stage pressure shut off compressor, one that shuts off the first stage at the 90 PSI, but the second stage won't shut off until it reaches 175 PSI.
Another factor to take into account is the “cubic feet per minute” or CFM. Most air tools will require 4 to 6 CFM. At 90 PSI you will be getting 3 to 4 exact HP. CFM tells you how much air is being moved.
Tank size should also be taken into account. A regular pump and motor with a normal 30 gallon tank is usually fine if what you are doing only needs to use short bursts of air. But for heavy use, you are better off using a smaller tank with a significantly stronger motor and pump, with attentive consideration to the stored air in the tank.
Lastly, there are gas powered and electrically powered compressors. If you know you will be working in an area without access to electricity, obviously the gas powered compressor is the way to go.