Impact drivers are most similar to powered drills, however they still have the impact features to it. They have a 6 point insert in them to hold drill bits or screwdriver bits. This is great for driving screws into wood or other materials.

Impact wrenches are going to be for tough, metal on metal work. They have a 1/2", 3/8", or 1/4" head for sockets. 

This mistake seems simple, but its not always completely clear on the box. Be careful!