What is the difference between a regular DVD and a BLU Ray DVD?

Both disc formats are the same physical size and shape. Beyond that, they are quite different. Many of the answers here are correct or partially correct. Here's a complete run-down:

- Blu-ray discs are read using blu lasers rather than red lasers. This is one of the main reasons they are completely unreadable by a normal DVD player.

- Blu-ray discs can hold up to 25GB (single-layer) or 50GB (dual-layer). Discs on sale today are a mix of 25 & 50GB discs, its difficult to tell from the packaging which is which, but both types work in all players, so its somewhat irrelevant to the consumer.

- Blu-ray movies can be encoded in MPEG-2 (like DVD) or newer compression systems like MPEG-4 "AVC", or Windows Media "VC-1" format. Each has their pros & cons, but the newer codecs generally compress data more efficiently.

- Blu-ray movies support additional audio formats including Dolby Digital (same as DVD), DTS (same as DVD), Dolby Digitial Plus, DTS-HD, DTS-Master Audio, as well as uncompressed PCM encoded audio tracks. Most of these formats support higher bit-rates and sizes than traditional DVD (so basically, they sound better).

- Blu-ray movies have additional layers of copy protection compared to DVD, so theoretically the "DeCSS" fiasco which made all DVDs easily pirated does not / will not exist on Blu ray.

- Blu ray packages are smaller and thinner than DVD cases and are translucent blue on the top and bottom, in an effort to avoid consumers buying the wrong discs.