Blu-ray discs are identical in size to DVDs, but can store much more information so they are used to deliver high definition video and audio. The "Blu" refers to the wavelength of the laser used to read from the disc, which is much shorter than the red wavelengths used in DVDs.
Blu-ray discs require a Blu-ray player and high definition television set. They cannot be played on DVD players.
However, DVDs (and CDs) can be played on Blu-ray players, protecting your investment in DVDs which will be upscaled to enhance picture quality. They will also play Blu-ray Audio discs, which give much higher sound quality than CDs, and many also play SA-CD discs.
Most Blu-ray players also provide apps for on-line services such as BBC iplayer, ITV Hub, STV player, Netflix, and other catch-up services, which some high definition television sets may not have.