TWAIN is a widely-used program that lets you scan an image (using a scanner ) directly into the application (such as PhotoShop) where you want to work with the image. Without TWAIN, you would have to close an application that was open, open a special application to receive the image, and then move the image to the application where you wanted to work with it. The TWAIN driver runs between an application and the scanner hardware. TWAIN usually comes as part of the software package you get when you buy a scanner. It's also integrated into PhotoShop and similar image manipulation programs.
The software was developed by a work group from major scanner manufacturers and scanning software developers and is now an industry standard. In several accounts, TWAIN was an acronym developed playfully from "technology without an important name." However, the TWAIN Working Group says that after the name chosen originally turned out to be already trademarked, an 11th hour meeting of the group came up with TWAIN, deriving it from the saying "Ne'er the twain shall meet," because the program sits between the driver and the application. The name is not intended to be an acronym.