Live rock is generally rock gathered from a marine environment (most often reefs) used in reef aquariums for a variety of purposes. It is generally very porous in nature, formed from the old dead skeletons laid down by corals, coralline algae and other calcifying reef organisms. The label "live" is applied because of the multitude of different organisms that are present on and within the rock. Manmade liverock is also possible, but is typically "seeded" with reef collected rock to introduce the organism.
Along with live sand, live rock forms a large part of the biological filtration required in a reef aquarium - it plays host to various bacteria which participate in the nitrogen cycle, as well as various detrivores, worms and other creatures that aid in the removal of waste.
Liverock can also be considered to be decorative - attractively shaped pieces of rock which are considered desirable for aquascaping a reef aquarium. A marine aquarium looks very bare without some form of rockwork.
Liverock also forms the platform for corals and other sessile creatures to be placed on, as well as forming a hiding place and / or home for more mobile livestock which live amongst rocks in the ocean. Many fish like to hide/swim amongst rockwork, and invertebrates such as shrimps and crabs often live in rocky areas.
Live rock can be obtained in either cured or uncured states. Which of these is appropriate to use depends on the system.
Cured refers to liverock that has been stored in a tank for a significant amount of time between when it was collected from the ocean and the time it was purchased. During this period, most of the die off has taken place already, which means that it can be added to tank containing fish, with care. The die off occurs due to the fact that some organisms were damaged during the collection, transportation and storage of the rock, or they are simply not suited to the captive environment.
Uncured liverock is fresh from the ocean and will typically have at least a small amount of die off after it is obtained. The amount of die off depends on the handling and time taken to transport it back to the system it will be placed in and the conditions provided for it. It has been known for people to get fish (blennies come to mind most) on the live rock.
If uncured liverock is used in moderate or large quantities relative to the system it is being added to, it has to be cycled. This is performed in a separate tank (if being added to an existing system or the target system if it currently contains no livestock) with no other livestock present. Ammonia, nitrite and nitrate need to be monitored to see how the cycle is proceeding. The lower that these parameters are maintained during the process, the more stuff is likely to survive on the rock. Conditions are maintained by performing water changes, having a well tuned protein skimmer and good water circulation. The more care that is taken during this step, the more amazing inhabitants will be found in the tank, which improves the overall system stability.
Base rock can be any inert and (preferably porous) rock or other material. Base rock can also be live rock that has little or no organisms populating the surface of the rock. Porous but otherwise dead rock will populate with micro organisms eventually and become part of the filtration system.
Some retailers label their rock as being "Premium". What this typically means is that they have a significant amount of life on the rock, such as coralline algae, macro algae, zoanthids, corals and various other organisms.
White "cobwebs" or "fungus" is due to organisms on the liverock dying during cycling.
How much live rock do I require?
For the majority of cases there is no correct amount of liverock to add to provide sufficient biological filtration. The rules of thumb typically seen thrown around on the mass of liverock per tank volume are of no real use. What it comes down to is the appearance that is being aimed for with the tank overall and therefore the reef structure to be constructed. The liverock has a large surface area available for bacteria to colonise, so even what looks like not much will be able to support enough bacteria. Additionally, if any sand is used within the system (including the sump and/or refugium), then that can contribute a significant amount to the surface area available. Something that is equivalent a quarter of the tank volume or a third of the visual area can provide a great looking tank with plenty of space for fish to swim and growth of corals. But that is personal preference and others may be looking more for a wall of rock look, less with a bommie surrounded by a large area of open sand or something in between. For information on types of reef structures, see aquascaping.