Quoted from Reef Invertabrates, An Essential Guide to Selection, Care and Compatability
by Bob Fenner and Anthony Calfo
"Live Rock, by definition, is a mixture of a hard, non-living mineral matrix with an assemblage of diverse living organisms on, in, and amongst it. Nearly all living phyla of monerans, protists (including micro and macro algae, protozoans and more), plants and animals make up this stony "bouillabaisse". One need only look closely to make out much of this life; scurrying amphipods, sprouting plants and algae, coral larvae, boring clams (spitting "smoke" as they mine a calcified residence), urchins and countless other organisms."
, just answering the question in the title
Get the pics and we can get a little more specific.