While traveling around the country as a trainer, I realized there was a lot of confusion about the difference between hammer drills and rotary hammers. Let me get something straight here, they are very different tools.
A hammer drill uses a percussion mechanism to generate a small impact force, but does it at super high speed. A rotary hammer uses a piston – striker – impact bolt – bit setup, which generates a lot more impact force, but does it a lot slower than a hammer drill.
Hammer drills have a 3 jaw chuck and use round shank bits. Many are cordless, small impact force, high BPM, and great for things without hard aggregate, typically not for concrete.
Rotary hammer have either a SDS Plus, Spline, or SDS Max chuck and use the corresponding bits. They have high impact forces, and are great for concrete.