There are 2 major skin formats supported by WindowBlinds/Skin Studio: UIS0* and UIS2.
UIS0* is a skin format used on Windows Vista and 7 that takes advantage of the Aero themes included with the OS itself, but allows users to customize this in a simple, but highly compatible way.
UIS2 is the format most commonly used for skins, and is what most people think of when they think of WindowBlinds skins. This format allows the skin creator to fully replace visual elements of the Windows UI, allowing for much greater flexibility.
* UIS0 is no longer an option in Windows 8, therefore WindowBlinds 8 only contains UIS2 skins.