The compressor is a machine that compresses the gases in the air around us to pressures greater than that of the normal atmosphere. Most of us first made the acquaintance of a compressor when our bicycle tires needed pumping up, and the basic mechanism hasn't changed all that much.
The compressor consists of a motorized pump; a tank for storing the compressed air; an on/off control (governor) that tells the pump when to start and stop in order to keep the pressure within preset limits; and a regulator to control the pressure at which the air escapes the tank to suit the needs of the tires (or the tools) being serviced. There's a metal frame on which all the rest of the parts are mounted, perhaps a handle and, in some cases, wheels. And various pipes, gauges, nozzles, cords, and other hardware.
The big difference between the service station compressor of your training-wheel days and the present is the portability and widespread use of compressors for many different kinds of workshop (and worksite) tasks. You would have to be a descendant of Rip Van Winkle not to know about nail guns. And spray-paint attachments, sanders and a dozen other kinds of attachments.
Not every workshop needs a compressor, but a busy shop can be made more efficient with the addition of compressed air. An air line with a plain nozzle on the end is handy for blowing sawdust out of a workpiece or any number of other tasks. Not to mention the efficiency of some of the pneumatic tools.
Probably the greatest advantage of air-powered sanders, drills, and other tools is their light weight. That makes them easier to use (and less tiring to hold), especially when you are performing repetitive production tasks. They are also compact and don't heat up (there's no armature and motor inside). Since they have fewer moving parts, pneumatic tools require less maintenance and are less likely to break down.
The sales talk for compressors involves a confusing array of technical terms. The key ones both concern the output of the compressor. It's measured in how many standard cubic feet per minute of air (scfm) are delivered at how many pounds per square inch (psi).
Various machines require differing volumes (scfm) at different pressures (psi). Before buying a compressor, you must identify the tools you plan to use, and match the compressor's output to the pressure requirements of the tools. Most tools required three or four scfm at ninety psi, though sanders typically demand more volume.
Another consideration in selecting a compressor is portability. Many compact models can be carried (with some effort) by one person to a worksite, an essential requirement if you plan to put the compressor to use in framing your new workshop. On the other hand, if it is to be set in place in your workshop for good, buy a stationary compressor. Two-stage models (in which the air is compressed twice with an intermediate stage in between for cooling) produce higher pressures more efficiently and, typically, last longer. The trade-off is in expense and weight.
Check the oil at least weekly; change it frequently (check the manual for the manufacturer's recommendations). Use the weight oil prescribed. Empty the air tank frequently, then open the drain tap at the bottom of the tank. This will allow any water that has condensed to drain off before any of the machine's components corrode.
Accessories. The list is long and increases every year as more and more uses are found for compressed air, and the price of components becomes more affordable. The random-orbit sander is a tool that is available in a air-powered model. There are the ubiquitous construction nail guns seen at work-sites almost everywhere. At the local tire shop, you've seen the impact wrenches that handle lug nuts as if they were M&M's.
And that's only the beginning. As well as random-orbit sanders, there are straight-line and orbital sanders. There are drills, some with variable speeds and reversibility. All sorts of pneumatic grinders are sold, including die, pencil, and vertical grinders; there are angle cutters, too. Carving hammers and ratchets are joined by laminate trimmers and saws, including jig, circular, panel, even reciprocating saws. Spray equipment for finishing can also be run off compressors.
An equally wide range of nail guns are sold for specific purposes (no one gun accommodates more than one kind of nails). Roofing, framing, finishing, and brad nailers, as well as crown staplers are on the market. Sand blasters, power washers, and even drain cleaners are also available.