A few years back (2006-2007) there was a huge "battle" between the two, the blu-ray had better picture and audio but the HD was cheaper, eventually Toshiba (the creator of HD-DVD) ceased production and now makes Blu-Ray players.
So basically they dont make HD-DVDs anymore, and Blu-Ray has better picture and sound
Some say "HD" technology parallels "Blu-ray"; However, "HD" has been phased out and is no longer supported. Blu-ray is the current standard in high definition viewing.