An impact driver is a tool that delivers a strong, sudden rotational force and forward thrust. It can be powered either by striking with a hammer in the case of manual impact drivers or mechanically in the case of powered impact drivers. The tool is designed to deliver high torque output with minimal exertion from the user. It is more compact and lightweight than a power drill and has more torque than a power drill. Impact drivers are available in cordless designs ranging from 12 volts up to 20 volts, and as the voltage increases, so does the power and speed of the impact driver.

An impact driver spins a bit and uses an internal mechanism called a hammer and anvil to turn whatever you load in the collet (the impact driver version of a chuck). The hammer strikes the anvil, creating an impact and forcing rotation. Impact drivers are more commonly used to drive in a large quantity of fasteners, longer screws, and lag bolts. Long screws and, with the use of an adapter, lag bolts can be driven in more easily by an impact driver. If you use impact-rated, 1/4-inch hex-shank drill bits, you can accomplish some basic drilling tasks with an impact driver. However, these tasks will be limited in scope.

Impact drivers are superior at directing their force downward, making them more comfortable to use over a long period of time. The added force offered by the tool saves your hands and wrists as you work. This can help extend the time you can comfortably work on your project. Also, the impact driver is much less likely to strip your screws when in use. The short pulses tighten in a more controlled fashion.

In summary, an impact driver is a tool that delivers high torque output with minimal exertion from the user. It is more compact and lightweight than a power drill and has more torque than a power drill. Impact drivers are commonly used to drive in a large quantity of fasteners, longer screws, and lag bolts. They are superior at directing their force downward, making them more comfortable to use over a long period of time, and are much less likely to strip your screws when in use.