HDCD stands for "High Definition Compatible Digital" and was developed by a company called Pacific Microsonics, Inc back in the late 90's to improve the overall sound of conventional CD's . The technology was then acquired and purchased by Microsoft who now licenses the technology to specific record companies who choose to produce their CD's in this format. Sound is greatly improved by packing more "information" into conventional CD's with a standard sampling frequency of 44.1kHz and 16 bits per sample. CD players which do not decode HDCD will produce somewhat improved sound of an HDCD CD but those players which do decode it will sound far better!! I know. I have the Rotel RCD-1072 CDP which does decode and I'm building my CD collection of HDCD's. They are without a doubt far more enjoyable with greatly improved soundstage, bass depth, detail, etc. Certain artists such as Joni Mitchell and the Grateful Dead are reproducing their CD's in this format. It's especially useful on the older recorded albums. You'll also find some smooth jazz artists with HDCD's such as Dave Koz and Brian Culbertson using this format on "some" of their CD's.
DTS stands for "Digital Theater System" and is a format used in Surround Sound receivers and processors. It's a competing surround sound format to Dolby Digital. If a DVD is formatted in DTS and you play it back in DTS, it will sound its best. Honestly, I've never heard of DTS CD but my guess is that the music on the CD is encoded in DTS so it would be best to play it back using a surround sound receiver in that format. Gold and XRCD I'm not familiar with. Another competing high def audio format now is called SACD (Super Audio CD's). These are supposed to sound superior to even HDCD's and the players are generally more pricey than those with HDCD. The Marantz SA8003 CD player is one good example.
