Live rock usually has coraline algea and all sorts of live stuff on it, sponges, clams/oysters, corals (although often times these things are dead when we get the rock) that comes from being left in the ocean. Base rock is the same kind of rock, but has none of the stuff that make live rock 'live'. It's usually white with nothing growing on it. It's good for making foundations, and putting more decorative pieces of LR on top of it.