There are many types of Rock you can get, Dry rock, which was talked about earlier, Live Rock from a LFS, (most of what I see is wet dry rock or Import rock) and Aquacultured Live Rock.
We have top quality Aquacultured Live Rock, this is rock that has been placed in the ocean on a spot designated by the National Marine Fisheries for this purpose and let set for years, this creates a reef and it is inhabited by all sorts of marine creatures. Some people do not like the creatures and they choose to use dry rock, however I would suggest my rock or any Aquacultured Rock if your new to the hobby, if for no other reason than the learning experience you get trying to identify all the creatures.