If you've ever walked by a construction site and seen the guys drilling through concrete or stone, you've probably seen a hammer drill in action. Hammer drills work like a normal rotating drill except they have a "hammer" mechanism in them. This hammer pounds the back of the rotating drill bit allowing you to drill through harder materials.
Every hammer drill has a bit and a hammer. The bit can be a chisel or any number of drill bits. The hammer can vary in size and power depending on the size of your drill and whether or not its corded or cordless. Cordless hammer drills are not dedicated hammer drills and therefore best for simple home projects. They work well for drilling small holes in brick or concrete while larger block and stone require a larger, corded hammer drill.
One of the most common larger versions in the rotary hammer. These have a hammer-only setting on them which is great for harder jobs like chiseling up tile floors, drilling larger holes in stone, and breaking up masonry. These are larger and more powerful hammer drills but also overkill for most home uses.
The smaller, 18 Volt handheld and battery operated drills with the hammer function are far better for home use. A true hammer drill is not for drilling in screws. It's strictly for drilling holes and chiseling. The handhelds on the other hand can be switched out of hammer mode and used to actually put screws in after you've drilled the hole. If you're a DIY'er, handyman, or just a normal guy or gal who has stuff to do around the house, you should always have a dependable drill like this.
If you have a tile floor to pull up or need to drill a hole through your block wall, then you probably want to use the larger rotary hammer drill. These are corded, have a hammer only mode, and the most powerful. But if this is a single project you should just rent one or borrow one from a buddy.
If you're a dedicated DIY'er and aren't doing the things I just mentioned, then get the cordless 18 V drill with a hammer setting. These can be used to drill smaller holes up to about 1/4" through brick, some stone, and other comparable substances. You can also switch them out of the hammer setting and use them to drill through wood, tighten or install screws, metal, etc.