My wife got me a big honkin' Husky 125 psi 6HP unit for our wedding anniversary a few years ago! What a gal!
The compressors have single (or double) piston pumps that are belt driven by an electric motor. The smaller units are 120V, but the big stand-up models require 220V! Fortunately I had the garage wired for it in anticipation of getting one someday!
The pumps require oil and the big tanks should be drained regularly, as moisture builds up inside and could rust. I've seen some shops with a drain system, and I plan on doing that one day.
So far I own a (cheap) air ratchet and a gravity feed spray gun, but at best, the compressor is really a glorified soccer ball and pool toy inflator! :p
I would recommend that if you plan to purchase air tools, get the best you can afford. My cheapie ratchet has no "grunt" whatsoever, so I'm torqueing and loosening with my own strength and then just letting the tool spin the nuts off (just so I can hear it making the noise and impressing my neighbors).
Also, if you get an impact wrench, you need to invest in a set of impact sockets (gunmetal black ones)...can't use the regular (chromed steel) ones or you'll crack them in no time!