About reciprocating air compressors:

Reciprocating air compressors are positive displacement compressors. This means they are taking in successive volumes or air that is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element.

The reciprocating air compressor is considered single acting when the compressing is using only one side of the piston. A compressor using both sides of the piston is considered double acting.
The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exits across the valve. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure.A compressor is considered to be a single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ratio (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature and other design problems.

There are a few kinds of reciprocating air compressor:

1) Oil less compressor
Oil less reciprocating compressors are designed with a low-friction coating on their moving parts to eliminate the need for oil, are generally less expensive than their oil-lubricated counterparts and do not require you to change the oil on a regular basis.  And, the air they produce is cleaner because there is no residue from the lubricating oil (although you should still use an inline filter).  These compressors are usually noisy (89 decibels and higher), and generally have duty cycles of 50% or less.

2) Piston reciprocating oil lubricated
These compressors use oil for lubrication. They're significantly quieter than most oil less compressors and, if taken care of, can last for a very long time. You will need to change the oil periodically, and you'll need to install a high quality inline filter because of small amounts of oil that get in the air. Life expectancy generally runs anywhere from 2,000 to 6,000 actual pumping hours between rebuild.  Both single and two stage units are available; they generally feature all cast iron construction for durability and noise reduction.

3) Two stage compression
Two stage compression units will hold much more air at a higher pressure. You're essentially squeezing the air in tighter. Effectively, there can be as much as 140 percent more air delivered. For example, an 80-gallon tank and a cut-out pressure of 175 psi, has the equivalent air storage of a 194-gallon tank!  The "cut-in" pressure (the pressure level that kicks on the compressor), in compressors currently on the market, is about 95 psi, while the 2-stage compressors have a cut-in pressure of about 145 psi, so the compressor keeps a significant reserve for long blasting jobs.