Marietta, basicly your options fall into three catagories: motar bed, tile backer, other (eg. plywood). A mortar bed is the most expensive, and most would agree, the best. The most commonly used types of tile backer bd. are: cement bd. (Durock is one of many brands), Hardibacker, and Den-Shield.
Cement bd. is composed of cement and fiberglass, is manufactured to meet or excede ANSI A-119.9 specifiation, and is recognised by the TCA (Tile Council of America). It is impervious to water, but not water proof, and available in 1/4", 1/2", and 5/8" thicknesses.
Hardibacker is a proprietary composition of cement, sand, and cellulose fiber. It is available only in 1/4". While not recognised by the TCA, it is recognised by several building codes. It has not been around as long as cement bd.
Dens-Shield is genericly referred to as glass mat water resistant gypsum backer bd. (it bears no relation to other types gypsum board such as green board), and is recognised by the TCA. It is impervious to water, and waterproof. Additionally, it doesn't wick water the way cement board does. It is available in the same thicknesses as cement bd. You can find more details at: www.gapac.com/build/gypsum/dens/tilebacker.html.
All of these backers bd's are non-structural and require their sub-flooring to meet certain minimum spec's. When properly installed, all give good service.
When you talk about waterproofing, there are various degrees, and you should tailor the installation to the expected usage. All of the backers can be waterproofed (though not to the degree of a shower pan); the advantage of Dens-Shield is that you only have to worry about the seams and edges.
You can get a TCA Handbook at www.tileusa.com for $3 which shows many proven installations methods.