What's a two-stage air compressor?

While shopping for a home-shop compressor, I saw an online listing for a "two-stage" machine. Would it be appropriate for running nailers and a gravity-cup sprayer? What's the difference between a two-stage and a regular compressor that, I guess, is single-stage?
—Jeff Burgess, Corvallis, Ore.

Unless you plan to do large-scale spray painting (homes, cars), pneumatic sanding, or remove wheel lug nuts on a vehicle, Jeff, you should not need a two-stage compressor. Here's why. A single-stage model (above left) draws air into its pump and then compresses it directly into the tank. This type of compressor runs on 110-volt current, ranges in tank size from 1 to 60 gallons, and produces up to about 11 cubic feet per minute (cfm) of airflow. Furniture-scale spray guns require 7–12 cfm, depending on model.

For most home woodworking shops, a two-stage unit, which requires 220 volts, is overkill. This type, above right, has two pistons, housed in a cast-iron pump, to divide the workload. The first piston compresses air to about half the desired pressure level, then sends it through an intercooler (removing built-up heat) to the second cylinder, where the air gets further compressed and then sealed inside the tank. These compressors usually have 80-gallon or larger tanks and create about twice

the airflow of single-stage units. This allows you to use high-demand tools, such as large sprayers, impact wrenches, and dual-action sanders, without running low on air pressure.