Check out this decent (but as usual somewhat incomplete) Microsoft page on STI, WIA and TWAIN.

STI (STill Image Interface ?) and WIA are Microsoft-defined APIs. STI is not a full API for image input, Microsoft calls it a "low-level hardware abstraction" and it seems to function mainly as a way of providing push-model imaging, where the user pushes a button or does something with the physical imaging device, and triggers the launch of an application and capture of the image. STI doesn't define how the application actually brings in the image (it could be WIA or TWAIN), it just provides the signaling path from the button to the action. Many flatbeds now have one or more buttons that activate software through STI.

WIA: "The WIA architecture enables imaging applications running in user mode to communicate with and receive data from imaging devices, such as digital cameras and scanners." - Microsoft. WIA is a superset of STI. WIA is very much competitive with TWAIN, with much better support for digital cameras, and much less sophisticated support for all other types of devices.

TWAIN is an industry standard API for image input - we have a Dosadi page about it. The primary source is the TWAIN Working Group.

STI is supported on Win95 (by SP), on Win98, and Win2K.

WIA is supported only on WinMe and WinXP.

TWAIN is supported on all versions of Windows, on Mac OS (8, 9, and X), OS/2 (!). TWAIN 2.0 (Spring 2005) will add support for Linux and Unix, and 64-bit Windows.