HD DVD, or the High Definition Digital Versatile Disc, was an optical disc format developed by Toshiba in the mid-2000s as a successor to the DVD format, specifically with the growing popularity of High Definition content in mind. The format was born out of disagreements within the DVD Forum, which was in the process of developing the Blu-ray format, whose earliest prototype discs required a protective caddy to keep the disc from getting damaged easily. A faction within the Forum was unhappy with this requirement (which would eventually be absolved when Blu-rays finally came to the shelves) and decided to create their own competing format that was more similar to the classic DVD, and eventually splintered off into its own organization.
The discs are visually and physically very similar to the other optical formats, having the same measurements as a standard 12 cm CD, DVD or Blu-ray. Specifications wise, it shared a lot in common with its main competitor, using short-wavelength blue lasers to read and write to the discs, and supporting a lot of the same video and audio codecs, as well as supporting content with a resolution up to 1080p (Full HD). However, the format was weaker than the Blu-ray in many other aspects. The discs had a maximum storage capacity of 15 GB per layer as opposed to the Blu-ray's 25 GB per layer, and had a generally lower maximum bitrate for both video and audio. One side where the HD DVD shines is in its total lack of region coding, meaning that any HD DVD will play on any HD DVD player, regardless of where in the world it belongs to. Attempts were made by Disney to add region coding to the discs, but it was met with resistance from everyone else.
The first HD DVD players from Toshiba, as well as the format's launch titles, released on April 16, 2006. The format launched with four titles: The Phantom of the Opera, Million Dollar Baby and The Last Samurai from Warner Bros., and Serenity from Universal. Several models of HD DVD players were released, including both internal and external PC drives and an external player compatible with the Xbox 360, which only supported DVDs out of the box. Like with Blu-ray players, all HD DVD players were backwards-compatible with CDs and DVDs.
By now, you've probably figured out how things went from here on. The competition between HD DVD and Blu-ray eventually resulted in a major format war with multiple companies on each side, with HD DVD supporters including Universal, Paramount, Warner Bros.. and eventually DreamWorks Animation, and Blu-ray supporters including Sony Pictures (naturally, as their parent company was involved in the making of the Blu-ray format), Disney, and 20th Century Fox. The format war started shifting quickly in Blu-ray's favour, however, as its more advanced DRM options eventually lured Warner and Paramount over, cinephiles started touting its superior audio and picture quality, and many consumers in general also seemed to prefer the Blus as well despite HD DVD being generally cheaper, leading to Blockbuster eventually gutting their entire HD DVD stock altogether after discovering that 70% of their HD rentals were Blu-rays. Eventually, some studios started supporting both formats, and 2-in-1 players that supported both formats were made. Warner Bros. even commenced development on a hybrid disc format titled "Total Hi Def", a two-sided disc that bore a HD DVD on one side and a Blu-Ray on the other, although it was ultimately never made.
Another advantage for the Blu-ray came in the form of the PlayStation 3. As mentioned above, the Xbox 360 required an external add-on to play HD DVDs, and it never ever used them for games, rather sticking with plain old DVDs. Meanwhile, the PS3 supported Blu-rays natively AND used them for its games, meaning that a lot of gamers automatically had a Blu-ray player in their home, and developers also liked to take advantage of the extra storage space; as an example, Metal Gear Solid 4: Guns of the Patriots released as a PS3 exclusive due to its large amounts of cutscenes and voice clips necessitating the extra space, and Final Fantasy XIII became a Multi-Disc Work on the 360 despite fitting on a single disc on the PS3.
The decisive moment came when Walmart decided to stop stocking HD DVDs and players in early 2008 after several studios had moved exclusively to Blu-ray, which was regarded as a fatal blow to the format since the company was the largest home video retailer in the US at the time. Several other retailers and Netflix followed suit, with most having cleared out all of their stock by the end of the year. Toshiba saw that the writing was on the wall, and announced their end of support for the format in February of that year, and the studios who were still releasing titles by that point followed suit and started backing the Blu-ray format exclusively for hi-def content. In 2009, Warner Bros. even issued a replacement program where they would trade in previously bought HD DVD discs for Blu-ray copies of the same titles.
As a side note, the last major studio titles to be released on HD DVD in the United States were Into the Wild from Universal and P.S. I Love You and Twister (1996)note from Warner Bros., both in May 2008. The last wide-release HD DVD overall was the sixth episode of Freedom Project from Bandainote in June of that year, and the format's final overall release ended up being a Germany-exclusive special steelbook release of Death Proof in December of that year.
These days, the format is remembered primarily for its failure, as the Blu-ray practically outdid it in all factors except price, and those who wanted to watch movies on the cheap were generally fine with regular old DVDs anyway. Discs and players can of course still be found floating around thrift shops and the like. An easy way to spot an HD DVD is by the red plastic cover as opposed to the blue cover that Blu-rays usually come in. The format has no exclusives, however, so the only real reasons to purchase and watch HD DVDs in this day and age are for collector purposes or just the sheer novelty factor of running a dead format. Then again, the discs are still very much watchable, so if you still have some lying around and a compatible player, you can't really go wrong with them unless you're a serious cinephile who can't do without the slightly improved video quality of a Blu-ray.
Interestingly, the format got a Chinese offshoot called the China Blue High-definition Disc (CBHD), which was developed as a home-grown Chinese alternative to HD DVD and Blu-ray, mostly so that Chinese manufacturers of discs and players could avoid paying royalties to foreign companies.note The format shares the same basic technology and has the same amount of storage space as a regular HD DVD, but uses its own video codecs and copy protection, all of which were made with government backing. Interestingly, development was assisted by Hisashi Yamada, an ex-Toshiba engineer who also played a role in creating the DVD format. CBHD discs became rather popular in China, outselling Blu-rays three-to-one, receiving support from major studios and eventually outliving its parent format. They are currently unavailable outside China, however.
Compare Betamax, which was locked in a very similar format war with VHS about thirty years earlier which it similarly lost.