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.
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.