When I think of an air compressor, I think of something that compresses a large amount of air through a small opening to create a large amount of pressure for using power tools. We have a small, portable air compressor that is used with a nail gun. We purchased it when building a fence, and it has really come in hand many times since. But I was recently looking for a new air compressor, and was overwhelmed by the different types on the market today. My main problem was that I didn't understand the difference between the different types. So I did some basic research and here is what I discovered.There are two main ways of compressing a gas, through positive displacement and through dynamics. Dynamic air compressors come in two main forms, centrifugal and axial compressors. Positive displacement compressors also come in two main forms, reciprocating and rotary. A reciprocating air compressor can be single-acting, double acting or use a diaphragm. A rotary compressor can be a lobe, liquid ring, screw, scroll or vane compressor. Now, most of these you do not have to know anything about, but if you are a layman, you can clearly see that there is a lot more to compressors than first meets the eye. For general use with tools, an air compressor is most likely to be a reciprocating piston, a rotary vane or rotary screw compressor. Reciprocating air compressors use pistons that are driven by a crankshaft. These types of compressors come in all sizes, from small and portable to large and industrial compressors. Some are powered by electricity, and others with an internal combustion engine.A rotary vane air compressor uses a rotor and a number of blades in radial slots in the rotor. I do not fully understand how this works, but it has something to do with the rotor turning and the blades sliding in and out of their slots as they keep contact with the housing wall. This somehow creates decreasing volume.A rotary screw compressor uses screws to force the gas into a smaller space. These can be quite compact or larger for industrial uses. The tighter the threads on the screws, the greater the compression. This type of compressor does tend to run a bit smoother and with less vibration than other types.As well as various types of compressors, there are two main types of pumps that are used in them, oiless and oil lubed. Oil free systems are a bit more technical so sometimes are more expensive. Often they are louder than an oil lubed compressor, but the air delivered through one of these pumps is of a better quality. They are found for home use, and are great because they are low maintenance and start up quick in cold weather.An oil lubed pump has a longer life with less wear than an oil free air compressor. These also come in various sizes and are great for home use. Though this information may not help you a lot in deciding what actual brand and model you should buy, it should help you to at least recognize basic terms. That way you will know the best questions to ask an expert and maybe understand better his answers. That way you can make the best decision according to your needs. After all, the right air compressor will make a lot of your home projects a lot easier and faster to accomplish.