HD DVD (or HD-DVD) stands for High Definition or High Density DVD. It is an optical disc format for storing high-definition video.
Since most types of DVDs used a blue laser with a shorter wavelength, HD DVD has the ability to store a little over 3 times as much data per layer. It allows a maximum capacity of 15 GB per layer rather than 4.7 GB per layer.
Giving rise to piracy issues, some subscribers to Dish HD and other HDTV services were attempting to record High-Def movies and TV onto writable HD DVDs.
As it stands now, it seems that Blu-Ray has surpassed HD DVD as a new standard for optical discs.