What's the difference between an impact driver and an impact wrench?

An impact driver uses a 1/4-inch hex collet and is built for driving screws and fasteners, while an impact wrench uses a square drive (typically 1/2-inch) for lug nuts and bolts. Several products marketed alongside impact drivers are actually high-torque wrenches; if your main job is deck screws, lags, and general fastening, you want a 1/4-inch hex impact driver like the DeWalt DCF845 or DCF787.