A VESA standard communications channel between the graphics card and monitor. The VESA Display Data Channel (VESA DDC) is used to pass Extended Display Identification Data (EDID), which is stored in the monitor and describes its characteristics (vendor name, serial number, frequency range, etc.).
The DDC requires an additional wire in the cable. The first level implementation (DDC1) provides a unidirectional channel that lets the monitor inform the host of its capabilities. A second bi-directional level (DDC2) allows the host to adjust the monitor. For example, the monitor's switch settings could be put into a software control panel. See
VESA Display Power Management Signaling and
VESA Extended Display Identification Data.