Impact wrenches are socket wrench power tools which apply a high amount of torque to tighten or loosen fasteners. They are typically pneumatically-powered but may also use hydraulic, manual, or (more commonly) electric power. Impact wrenches achieve high torque by storing energy within an internal rotating mass, then delivering it instantaneously to the output shaft.
Impact wrenches are very common tools in the automotive, industrial machine maintenance, and construction industries, although they can be applied to any industry which requires high-torque fastening.
A portable electric impact wrench.
Image credit: DeWalt