Live rock is a concept of saltwater fishkeeping that you will not see in freshwater. Basically Live rock is structure that adds to your aquariums decor but also does wonders for your biological filteration. Many smaller marine animals will use the live rock for shelter as well as a food source. Being structure, you can place your Marine frags on top of the rock to build a reef aquarium.
When your at the store you want to buy live rock that is fully curred. What this means is the rock should have red and purple growth on it. Plan brown rock is uncurred and you will either need to cure it yourself or not buy it. Live rock is usually cured in large containers called Vats. Under high powered compact lighting and daily water changes will cure the rock. Most live rock comes from Fiji. However you can find live rock from Tonga, Soloman islands, Marshal island, Caribbean. There are little differences between the locations, but to the novice saltwater fishkeeper, it will not matter.
Live rock is known as the second walet breaker, And thats for a reason. Most fully curred live rock will cost in the range of 6.99-13.99 per pound!!! The general stocking rule was 1-1.5-2 pounds per gallon.
Once and awhile you will find polyps or mushrooms on your live rock. Best thing to do is just leave it be and consider yourself lucky.
Live rock doesnt really need anything after you add it to the tank. You can break some pieces off of your live rock and it wont affect the health of the rock either. Just to finsh up some things... When getting live rock shipped to you, since water weighs so much, they will often ship it in wet newsspaper. You will have some die off but that is normal, a few days of curing will correct this. Also You can use live rock in sump filters. Basically since it does help with biological filteration, You can add it in your sump filter. Often called the berlin filteration method. Also in refugiums i would add live rock.