For the better part of the first half of the 2000s, two high definition disc formats fought for market supremacy: HD-DVDs and Blu-ray discs. Both formats had powerful backing from major electronics companies, but in the end Blu-ray emerged as the HD market standard, and as of 2011 HD-DVD is all but a memory. Blu-ray discs aren't like ordinary DVDs and you need a special Blu-ray player to play them. You can't play a Blu-ray in a standard DVD player.
The name looks to be misspelled, but according to the Blu-ray Disc Association, the "e" from "Blue" was left out to find a unique name for a registered trademark. Blu-ray players uses a blue-violet laser to scan discs, or a blue ray. The name is a combination of the two main technological components.
Blu-ray discs aren't compressed the same way as CD or DVD discs, which means that Blu-ray discs can't be read by your CD or DVD player. It's the same way that your DVDs don't play in your CD player: the formats are incompatible. But as technology develops, it is usually backwards compatible. DVD player manufacturers didn't account for a new technology so they couldn't design DVD players to be Blu-ray ready. Blu-ray manufacturers, however, knew all about DVDs and built Blu-ray disc players that can also read older formats, like DVD and CD. As a bonus, Blu-ray players up-scan DVDs to simulate HD quality -- though true HD video is video that was recorded in HD.
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The compatibility issue stems from pure technology differences. DVD players use red laser technology to scan for video and audio information on the disc; Blu-ray disc players use a blue laser to scan Blu-ray discs and DVDs, but the red laser in a DVD player can't read the information from a Blu-ray disc.
If you're sitting down to watch movies on an old, standard definition TV set, you will never see the difference between a Blu-ray disc and a DVD, even if you have a DVD player. On a 1080p HDTV and with a powerful sound system, the Blu-ray difference is clear almost immediately. But for something to appear HD on a Blu-ray, it needs to be filmed in HD or restored from film into HD.
The Blu-ray Disc Association has plans for a dual-format disc that contains DVD and Blu-ray data tracks, according to Blu-ray.com. These discs will play in DVD and Blu-ray disc players, but only using the proper format – so you still won't be able to put a Blu-ray disc into your DVD player and watch a movie in HD.