For a 25 gal
tank, you want about 25 lbs of LR
. The rock will become live through the cycle. It being live truely talks about the bacteria living on it that makes sure the ammonia and nitrites are consumed as they are produced. When you buy live rock, it means that it has been alive for some time and probably will be covered in different hitch hikers like worms, crabs, coral...whatever. Some are good, some are bad.
You will be able to tell that it is live by testing your water as you cycle your tank. So, if you don't have it already you will need a test kit. For ammonia/nitrite/nitrate purposes, an API master test kit will do nicely.