Nearly all image processing software today is TWAIN compliant. If your software supports TWAIN, you will find an "Acquire" command in the menus or toolbars (though sometimes the command is hidden under an Import menu). This command provides access to any TWAIN hardware devices installed on the system. Although the software appearance and capabilities for each device can vary, the TWAIN Acquire command calls up the hardware interfacing software, and places the acquired image into the image processing software, without the need for the image to first be saved to disk.
The word TWAIN is from Kipling's "The Ballad of East and West" - "...and never the twain shall meet...", reflecting the difficulty, at the time, of connecting scanners and personal computers. It was up-cased to TWAIN to make it more distinctive. This led people to believe it was an acronym, and then to a contest to come up with an expansion. None were selected, but the entry "Technology Without An Interesting Name" continues to haunt the standard.
-The Free On-line Dictionary of Computing, Editor Denis Howe