Although they are about the same shape and size, that's about as far as anyone can go with the similarities between the two. I found a list of differences online and will put it below.
- Blu-ray disks 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.
The rest is in the comments (stupid answer limit).
Blu-ray: Image quality, Superior resolution is a big part of what makes Blu-ray look great. Blu-ray: Audio quality is also improved. New high-resolution soundtrack formats, such as Dolby TrueHD and DTS-HD Master Audio, are essentially identical to the studio master, so you'll be hearing things exactly as the director and audio engineers intended. Blu-ray: Special features, Blu-ray also has additional special features over DVD. The most basic innovation is the pop-up menu, which allows you to access the menu while the movie continues playing. Other innovations include picture-in-picture video commentary and the ability to download new content right from your Blu-ray player.
It seems like blurays are significantly harder to damage. That and they can read and write at a higher level of quality, basically high definition. I don't think I've ever seen someone snap a bluray disk.