HDCD stands for High-Definition Compatible Digital or High-Definition Compact Disc. HDCD was a technology developed to improve the quality of audio CDs. A high quality sound system is needed to decode HDCD and take advantage of the improvements that HDCD promises. HDCD is currently owned by Microsoft Corp. Only a small selection of CDs are available that include HDCD.
HDCD was an attempt to increase the sound quality of audio CDs while also being backwards compatible with regular CD players. The format has died off however and doesn't even have information published on Microsoft's website. Instead, formats such as Super AudioCD (SACD) and DVD-Audio are preferred.
A HDCD encoded disc can be identified by a HDCD logo it would carry on its back cover. Windows Media Player 9 + with the Windows XP Operating System can decode HDCD if there is a 24-bit sound card available. It is the only available software decoder for HDCD. Since HDCD is proprietary, not a lot of technical information on the format has ever been released to the public, so it's claims of improvements on audio CDs cannot really be investigated.