This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
A The feature connector comes in two variants; VGA and VESA. The basic idea is that video memory can be directly accessed by using the feature connector, bypassing the CPU and peripheral bus. This reduces CPU load and avoids bandwidth bottlenecks. In addition, this eliminates the need for a separate RAMDAC. Typically, the feature connector is used by video capture cards or MPEG decoder cards, as it provides high bandwidth which is ideal for playing video clips. The older VGA feature connector is limited by the VGA itself, and can only display up to 256 colours at a pixel addressability up to 320x200. While this can provide reasonable images, they tend to look dithered, and the quality is less than that of NTSC television.
A more recent standard is the VESA Media Channel (VMC) which allows you to attach an MPEG or TV tuner card to your video card thorugh a high-speed connector. The VMC actually implements a full bus system which allows up to 15 devices to share the frame buffer and RAMDAC on the video card. More information on the VMC is available at the VESA WWW site at www.vesa.org, though the standards themselves are only accessible to VESA members.