What's the difference between a drill driver and a hammer drill?

A standard drill driver handles wood, metal, plastic, and screws. A hammer drill adds a percussive forward motion for drilling into masonry, brick, and concrete. If you're hanging shelves into drywall studs, a regular drill is fine; if you're anchoring into concrete or brick, mainstream reviewers consistently recommend stepping up to a hammer drill like the DEWALT DCD996B.