Impact wrenches deliver many times more torque than impact drivers, usually hundreds of pounds more. They are used professionally on heavy equipment and throughout the construction and automotive industry
Impact drivers, on the other hand, are increasingly popular for DIY and home projects. While they aren't as versatile as the likes of a cordless drill, they are great for building decks, sinking screws rapidly, general home repair works, and can even remove stubborn lug nuts (more on that later).
The difference between an impact driver and an impact wrench is, therefore, the difference between a driver and a wrench: One is designed to drive tough screws and fasteners, the other to tighten or loosen tough nuts and bolts.