HD-DVD, also known as Advanced Optical Disc (AOD), is a defunct optical disc format originally developed by Toshiba and NEC. HD-DVD was similar to Blu-ray (but not compatible) and used blue-laser technology to achieve a higher storage capacity.
The introduction of both HD-DVD and BD in 2006 started a format war similar to the Betamax/VHS war in the 1970s. Both were incompatible, and both had supporters and detractors. Blu-ray was arguably superior from a technological standpoint, but that means little because in these situations external influences such as politics, marketing, and overall industry support decides what will become the de facto standard. By 2008, it had become clear that BD was winning in overall market share, and this prompted several HD-DVD supporters to switch to the Blu-ray camp, thus ending the war. The decline of HD-DVD started near the end of 2007 when the largest U.S. video rental company (Blockbuster) declared it would only rent BDs. Then a major blow came in January 2008, when Warner Brothers announced it would not release new movies in HD-DVD, which started an industrywide chain reaction with several other studios following suit. The final blow came in February 2008, when Toshiba announced it would cease production of HD-DVD players, effectively ending the war once and for all.
Although a few combo Blu-Ray writable/HD-DVD readable drives (which also feature backward-compatibility with standard DVD and CD media) were introduced (the first combo drives feature LG's Super Multi Blue drive technology), HD-DVD players and discs quickly disappeared from the market after 2008.