An impact drill uses lateral percussion (from the side, in a circular motion). Once the drill senses that torque is needed, the drill uses impact to twist the screw into the surface. In short, an impact drill is used for driving screws and bolts that require a higher torque...
Impact drills are known for having a powerful torque that lets them drill through dense materials such as metal or knotted wood. Great projects for an impact driver include: Building a deck or fence. Assembling furniture with large screws.