reprinted courtesy of ServiceMagic.com
Cement board is a product that
revolutionized the tiling industry. It's literally a board of
concrete (usually in 4- x 5-foot sizes). It ranges anywhere from
a half inch thick for walls to 1/4 inch thick for floors. You
can lay it right on top of an existing subsurface to essentially
make it concrete. It resists shifting and is impervious to water.
Plywood is only good on floors and countertops where there are
no sinks or water sources. It's an acceptable subsurface for entryways.
-- Tips courtesy of Service Magic