Blu-ray is the high definition equivalent, and superseding technology, to DVDs. It was developed by Sony.
Consumer Blu-ray discs can store up to 25GB (single-layer) or 50GB (dual-layer) of data per disc. Higher storage volume is theoretically possible but not yet commercially available.
Standalone Blu-ray players and recorders are available. Additionally, Sony’s PlayStation 3 games console reads Blu-ray discs, and many laptop and desktop PCs have built-in Blu-ray read or read-write drives. Standalone Blu-ray drives for PC use are also available.
For some time, another competing high definition disc format — HD DVD — was developed and marketed by Toshiba. It was subsequently withdrawn from market.
Standard Blu-ray discs have the same physical dimensions as DVDs — 12cm diameter.
A wide range of high definition titles is now available on Blu-ray disc. Additionally, the specification is set to develop to encompass newly emerging 3D technology.
Much more technical information is available on the Blu-ray Wikipedia article.