the answerman's answer is INCOMPLETE; some HD DVDs also offer Dolby TrueHD &/or DTS-HD Master Audio...also, if you have an older receiver with MULTI CHANNEL INPUTs AND an HD DVD &/or Blu-ray player with MULTI CHANNEL OUTPUTs, you may use the players MULTI CHANNEL DECODER to enjoy these new formats AND Dolby Digital Plus & DTS-HD, both of which are superior to DD 5.1 and dts 5.1 found on regular DVDs...
new formats, in order of Quality, in my opinion
DTS-HD MASTER AUDIO 7.1 (LOSSLESS)
DOLBY TrueHD 7.1 (LOSSLESS)
DTS-HD MASTER AUDIO 5.1 (LOSSLESS)
DOLBY TrueHD 5.1 (LOSSLESS)
DTS-HD 5.1
DOLBY DIGITAL PLUS 5.1