So what is this stuff and why do you need it? Backerboard, or cement board, comes in sheets much like drywall and is used in a couple of ways. It is used on flooring installations as a means of creating a stable and easier to work with subfloor. It is also used in shower and tub enclosures (or anywhere with significant wetness issues) as a means of helping to create a moisture barrier. Unlike drywall, which is not water proof, or green board, which is merely water resistant, backerboard is water proof and when used with a proper moisture barrier help to mitigate headaches down the road.
Check out this ARTICLE by Andrew Hunt for more information about backerboard and how it is used.