Any HD content thats encryped I believe...I know on DVI displays it's optional although alot include it nowadays. Software programs and torrented movies, or non professionally distributed content I know for sure don't use HDCP, and I believe most normal DVDs don't either, the big one that uses HDCP is Blue Ray.
HDCP works basically like this. When a DVD or blue-ray disc is placed in a computer or DVD player, or when a satellite receiver is turned on, the transmitter immediately checks to ensure that the receiver is licensed with HDCP key controls. If the machine does not have the proper license in place, it will not be allowed to receive and display the HDCP-protected material. If the proper HDCP controls are in place, the DVD or audio and video content will be displayed on that computer monitor or television screen.
The HDCP license is checked for throughout the viewing session — not just the beginning of the transmission. In general, HDCP controls allow for friendly interchanges between the transmitter and the receiver, while blocking other receivers from intercepting, stealing, or otherwise viewing the content.