Blu-ray and HD DVD are rival incompatible formats, a situation that recalls the Beta versus VHS battle that stifled the early growth of the VCR and home-video market in the late 1970s and early 1980s. Despite an attempt to unify the two standards in 2005, the corporate godfathers of the two formats--Sony for Blu-ray and Toshiba for HD DVD--failed to come to an agreement.
Short for high definition-DVD, a generic term for the technology of recording high-definition video on a DVD. In general, HD-DVD is capable of storing between two and four times as much data as standard DVD. The two most prominent competing technologies are Blu-ray and AOD.
Blu-ray Disc (BD) - uses a 405nm-wavelength blue-violet laser technology, in contrast to the 650nm-wavelength red laser technology used in traditional DVD formats. The rewritable Blu-ray disc, with a data transfer rate of 36Mbps (1x speed) can hold up to 25GB of data on a single-layer disc and 50GB on a dual-layer disc. On a 50GB disc, this translates into 9 hours of high-definition (HD) video or approximately 23 hours of standard-definition (SD) video. The Blu-ray format was developed jointly by Sony, Samsung, Sharp, Thomson, Hitachi, Matsushita, Pioneer and Philips, Mitsubishi and LG Electronics.
What that means to you is that no standard Blu-ray player will be able to play HD DVD discs, and no standard HD DVD player can play Blu-ray discs (pricey combo players are the exception). If a movie comes out in one format, there's no guarantee that it will be available in the other. Certain studios could release movies in both formats, but you'll still have to be careful not to buy the wrong version of the movie.