Compressors fall into three general areas; Small portable units like the Porter-Cable "Job Boss" often with one pancake or two small storage tanks, Mid-size units like Campbell-Hausfield, which normally have wheels, and large compressors have fixed bases for shops and industrial sites. In terms of capacities, smaller units start around 1.5 horse and larger unites can exceed 6 horse power motors. The pumps themselves are either oil-less or oil-lubricated and the pumps are one or two cylinder and singe of two-stage compressors .... and this last statement is the reason most wood workers don't have an air compressor, because they sound complicated.
Some of the useful terms that you should know about when looking for a compressor include: PSI - Pounds per square inch, CFM - Cubic feet per minute, Duty Cycle - is the amount of time a compressor can run compared to the amount of rest time (storage tank) over a 30 minute period ... such as it can operate for 10 minutes, then rest for 20 without creating significant head wear, Single Stage - is where the compression piston compresses all the air in one stroke, Two Stage - is where there are two pistons that work together to compress the air. (Generally a two stage requires less horse power, and produces less heat).
As a rule, the types of tools that are harder on air volumes are those which run continuously like sanders, and grinders and cutters. Tools that require less cycling are tools that run intermittently like nailers, staplers and in some cases drills or even sprayers.
If you are planning on running some continuous tools like sanders, buffers, grinders and so on you will want to consider something in the mid-size or even large shop size of compressors like. In a case like this you should probably have a 25 gallon storage tank and around a 4 horse rating like a Makita or Craftsman. You will probably be looking at a 220 Volt version as well.
If you are planning on using a number of tools, some of them continuous draw, like grinders, sanders and buffers, you will want to consider a large fixed base compressor like Campbell - Hausfeld or Ingersoll-Rand. These will have around 6 horsepower and a storage tank of 50 gallons or more.
If you are a hobbyist woodworker, who is looking for a nailer or stapler, one of the small portable compressors is probably ideal. Manufacturers like Porter-Cable or Delta will work fine.
The real trick to buying a compressor is to know ahead of time what you are going to use it for and what the air-tool rating is in PSI and CFM. This will give you a better indication the duty cycle you will need. But this isn't end all either, because your need to look CLOSELY at the air tools you are considering because the same tool an vary widely in the amount of compressed air it will consume. For example a series of six inch random-orbital sanders have cfm differences of of 8 to 24. That's huge, and it means the difference between buying a mid-size compressor or large fixed compressor. It pays to examine the tools you will be using and select the appropriate ones.
A compressor and associated air powered tools can be a real time saver, even for the hobbyist's work shop. In many cases a simple brad nailer is almost like having a second set of hands around in assembling and gluing projects together. The other consideration in selecting a compressor is, what other uses will you find for it? For example if you do select one of the portable units, it is conceivable that you can carry this around to odd jobs, in the house, for friends or other areas. This is less of an option of the mid-size and out of the question for the large units ... but then the portables will not likely be suitable for sander/buffers ... SO ... in the end you will have to make that tough decision, but what ever you do, make sure you make sure get a nice selection of tools designed for your compressor. It will make your woodworking more fun, quicker and even more enjoyable than it is now.
