An impact driver looks similar to a power drill, but its only function is to drive or remove screws. It provides the same type of rotation as a drill but adds concussive strikes for driving large screws through hard wood.
Impact drivers use a different way to attach their screwdriver heads and sockets than power drills. Instead of a chuck, these devices use hex-shaped shanks on their accessories. They do not have the slip clutch found on the power drill. However, because of the concussive strikes, the torque of the tool transfers almost entirely to the screw, reducing the strain on the user's wrist. While similar to an impact wrench used by mechanics, impact drivers are designed for different uses.