They are all models for different markets and/or different providers or international versions. Some of them have 8mpix camera, some 13mpix, also if I'm not mistaken on some there is a microsd slot. Layout of camera, flash and speaker on backside is also different.
So differences in radios (regarding networks), cameras, card slots and layout practically means that you cannot use the same roms and cases on every model.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?