An impact driver is a tool primarily used for freeing sticky fasteners of one form or another. It appears at first sight to be a very chunky screwdriver with a large chuck into which one may place a screwdriver bit.
The handle of the impact driver is spring loaded and contains a wedge and cam arrangement. By hitting the impact driver with a hammer, the chuck is caused to rotate through a few degrees. The combination of shock, rapid onset of high torque and compression of the fastener's thread is particularly effective in freeing a seized, stuck or just plain annoying fastener.
The impact driver has a special advantage in use on crosshead,
Phillips and Pozidriv screws. As the impact drives the shoulders of the tapered screwdriver bit deeply into the head of the screw, it bites into the metal, helping to regenerate a damaged head and preventing stripping.
Many impact drivers are reversible, which is to say that they can turn in either direction when struck. They should never, never be used to tighten a fastener except as an immediate prelude to removing it, as the fastener may very well weld to the thread it sits in,
or the head detach entirely from the shaft of a bolt!.
The better impact drivers are fitted with a 3/4-inch drive, which means they fit into standard sockets. This is especially useful for removing engine head nuts. If the impact driver just isn't powerful enough, there are air-driven versions available which are capable of applying compression and torque far exceeding the tensile capabilities of metals.
spiregrain says: "how come I've never heard of these things?" because they're the reason the garage can undo that fastener you can't shift. Course they're not going to tell you about them... that'd destroy their nice little earner. Everyone should have one.