Firstly, most of the organisms that live on live rock are microscopic. Mainly bacteria and algae that are very beneficial - in a marine aquarium. You can also occasionaly get hitch-hikers on board which are larger than microscopic - worms, anemonies etc. If you have a freshwater aquarium however, anything that did live on the rock will, by now, have perished. The result is a lot of dead things that would result in ridiculous ammonia spikes and that could be disastrous if you have fish in the tank. Hopefuly, you have a saltwater tank - in which case, if the live rock was fully cured, your tank should now, effectively, be fully cycled and you can start adding your first fish. If it wasn't fully cured, you can expect some die-off now that you've added the rock and should monitor your levels until the tank appears fully cycled. Either way, please read up on cycling tanks. Also, please check out the saltwater/marine section of the forums - I think there's a pinned topic regarding live rock. Oh and there is no 'freshwater live rock' - the term 'live rock' only applies to marine rock with marine micro-organisms in it - sure there could be a freshwater equivalent but, for now, the closest you get is gravel carried over from an established tank.
