HDCP (wiki page) stands for High-bandwidth Digital Content Protection. It is a form of DRM designed to protect the digital data stream between the output device and the monitor. It is required for both Blu-ray and HD DVD, and all HDMI connectors are required to have it. More and more DVI connectors for PC video cards also has HDCP.

For Blu-ray/HD DVD, both your output device (eg. standalone player, PC video card) and your display device (monitor, TV) must have HDCP in order for protected HD content to be played back through the digital output.

For PCs, it is paramount to ensure both your video card output (usually DVI) and your monitor's input supports HDCP, or you may be limited to a SD picture or no picture at all. Those using dual-link DVI must also ensure it supports HDCP, because some older implementations of HDCP were limited to single-link connections.

More information about HDCP can be found in our High Definition DVD FAQ.