For a detailed look at the media and recording/ reproducing technology look up the Blu-ray association site They have general explanations, and technical details in greater depth. Unlike the DVD, developed by consensus, Shuji Nakamura is credited with the inventor of the first practical blue-violet laser that made the discs possible. The association has 19 board members which include computer heavy weights like Apple, Dell, Sun, Samsung, Sony and studio heavyweights like 20th Century Fox, Disney and Warner Brothers.
The Blu-ray discs also operate the same way CDs and DVDs operate. The exact same physical form factor allows for easy backward compatibility. The real difference is it uses a blue-violet laser that has a much smaller wavelength than the red laser used in DVDs. Compared to a DVD track pitch of 0.74 micron, the Blu-ray uses a track pitch of 0.32 micron. While the maximum pit length on a DVD was 0.4 micrometer; the maximum pit length on a Blu-ray is 0.15 micrometer. That directly affects its success in recording and reading more data on the surface of the discs. Use of better lenses help focus the laser on to the disc surface more sharply too and that helps. So much so that, compared to the maximum of 17 GB of data ( 9.4 being more common) of a DVD disc, a dual layer, double sided Blu-ray disc can accommodate up to 50 GB! That will hold a complete HD Movie and bonus material. Besides, this is a useful size for data backup uses too.
So the blue-violet laser bouncing off the disc surface has the capacity of reading/ writing more data in the same space as the pits representing the presence or absence of data are much smaller. Recording is made in the same spiral fashion on multiple tracks. Data read-off is then passed onto the electronic part of the player to decompress the data, convert to analog and take care of de-scrambling and the region code too. While discs initially needed a cartridge to protect them from scratches, a hard coating developed later provides enough protection. Cartridges are not used anymore.