An air hammer uses a weighted piston that is bounced from one end of a cylinder to the other with compressed air, hammering on an attachment, usually a form of chisel. A jackhammer is a very large airhammer. Most airhammers for garage/automotive use are small pistol-grip designs, like: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47868
Anything you can do with an air hammer, you can do using a normal hammer and handheld chisel, assuming you have room - but at 3000 blows/minute, an air hammer can typically do it a whole lot faster, and you just have to hold it rather than do any work...
