- A format used for high density video / storage discs, which allows for 4k videos to be stored and played back on a single disk. It is a format used for porn movies sold at retail.
Blu-ray or Blu-ray Disc (BD) is a digital optical disc data storage format. It was designed to supersede the DVD format, and is capable of storing several hours of video in high-definition (HDTV 720p and 1080p) and ultra high-definition resolution (2160p). The main application of Blu-ray is as a medium for video material such as feature films and for the physical distribution of video games for the PlayStation 3, PlayStation 4, and Xbox One. The name "Blu-ray" refers to the blue laser (actually a violet laser) used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used for DVDs.
The plastic disc is 120 millimetres (4.7 in) in diameter and 1.2 millimetres (0.047 in) thick, the same size as DVDs and CDs. Conventional or pre-BD-XL Blu-ray discs contain 25 GB per layer, with dual-layer discs (50 GB) being the industry standard for feature-length video discs. Triple-layer discs (100 GB) and quadruple-layer discs (128 GB) are available for BD-XL re-writer drives.
High-definition (HD) video may be stored on Blu-ray discs with up to 2160p resolution (3840×2160 pixels) and at up to 60 frames per second. DVD-Video discs were limited to a maximum resolution of 480p (NTSC, 720×480 pixels) or 576p (PAL, 720×576 pixels). Besides these hardware specifications, Blu-ray is associated with a set of multimedia formats.
The BD format was developed by the Blu-ray Disc Association, a group representing makers of consumer electronics, computer hardware, and motion pictures. Sony unveiled the first Blu-ray disc prototypes in October 2000, and the first prototype player was released in April 2003 in Japan. Afterwards, it continued to be developed until its official release on June 20, 2006, beginning the high-definition optical disc format war, where Blu-ray Disc competed with the HD DVD format. Toshiba, the main company supporting HD DVD, conceded in February 2008, and later released its own Blu-ray Disc player in late 2009. According to Media Research, high-definition software sales in the United States were slower in the first two years than DVD software sales. Blu-ray faces competition from video on demand (VOD) and the continued sale of DVDs. Notably, as of January 2016, 44% of U.S. broadband households had a Blu-ray player.
You can read more about Blu-ray at wikipedia here.