This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
[From: Dylan Rhodes (Formerly of Hercules)]
DCI stands for "Device Control Interface." It's an Intel/Microsoft standard, and exists primarily as a way for Windows 3.1 to exploit the video acceleration features of a graphics card, and/or to provide fast video when needed -- for example, the WinG games library uses DCI. A DCI driver exists at the same software layer as the GDI.
Among DCI's capabilities are the ability to write directly to the frame buffer (helpful for high-speed games) and the ability to provide for on-board hardware acceleration of video scaling (i.e. stretching a video window to a larger size) and color space conversion (converting the YUV format color information in a video file to the RGB format that a typical graphics card RAMDAC expects). Note that support for DCI features doesn't need to be in hardware -- a graphics card vendor could provide a DCI driver that allowed Windows 3.1 apps to speak DCI, but the graphics card could be performing the DCI functions with a software driver.
Note: with Windows 95, DCI will be replaced by an expanded interface called DirectDraw.