DTS CDs are music CDs that have 24 bit, 96 kHZ recordings compressed using the DTS codec. They will play on a DVD player if the player can decode DTS 96/24 or if connected digitally to your receiver and the receiver can decode DTS 96/24.
HDCD = High Definition Compatible Digital. It is an encoding technique for putting 20 bit, 44.1 khz recordings on a standard CD (which requires 16/44.1). When played in an HDCD compatible player or receiver with HDCD decoding, the player/receiver will 'decode' that signal as if the CD were actually using 20 bit samples. Think of it like the way 2 channel analog tracks can be encoded for ProLogic. They will play fine in a normal player, but one with a ProLogic decoder will know to extract the extra information. Same principle with HDCD. HDCD encoded CDs will play normally in any cd or dvd player - even if it is not HDCD compatible, you just won't get the supposed benefit of the higher bit depth.
All of the Van Halen Remasters are HDCD. I don't think they sound any better than the other non-HDCD versions I have, but as usual your mileage may vary.
