WIA and TWAIN
Check out this decent (but as usual somewhat incomplete) Microsoft page on STI, WIA and TWAIN.
Windows Image Acquisition (WIA)
"The Microsoft Windows Image Acquisition (WIA) API is standardized for acquiring digital images from devices that are primarily used to capture still images and for managing these devices." See this MSDN on WIA .
WIA is a COM interface, very different from but overlapping with TWAIN. Compared to TWAIN it offers much better support for digital cameras, and much less sophisticated support for scanners.
WIA is supported by all (desktop) Windows editions beginning with WinMe.
TWAIN is supported on all versions of Windows, on Mac OS (8, 9, and X), OS/2 (!). TWAIN 2.0 (as of 2009.05.22) is available in early release (read: beta quality) for Linux, Unix, and 64-bit Windows.