Hmm, that's kind of a trick in the descriptions.
Reefs are generally made out of ancient coral skeletons over millions of years. Largely, they are calcium carbonate, which becomes limestone, I believe.
"Rocks" are granite, quartz, silica, and others. Sure, if you go to the shore here in New England you can find huge boulders of granite or sandstone in the water. Do we use these in our tanks? Not usually in a reef tank.
Chunks of calcium carbonate "rock" from the reefs are usually what we use in our reef tanks. There are many types: Fiji, Tonga branch, Kwajalin, Marshall Island... etc. These names usually describe the origin of the rock, and maybe the form.
We can also get "dry rock" or "dead rock."
That's regular reef "rock" that has washed up on the beach and bleached and dried. There are a bunch of threads on dry rock if you want to search. It's nice stuff.