Blu-ray vs. HD DVD

The name Blu-ray Disc is derived from the blue-violet laser used to read and write this type of disc.
Because of its shorter wavelength (405nm ), substantially more data can be stored on a Blu-ray Disc than on the DVD format, which uses a red, 650 nm laser.
A single layer Blu-ray Disc can store 25 GB , over 5 times the size of a single layer DVD at 4.7 GB.
A dual layer Blu-ray Disc can store 50 GB , almost 6 times the size of a dual layer DVD at 8.5 GB.
The HD DVD disc is designed to be the successor to the standard DVD format, and is derived from the same.
It can store about three times as much data as its predecessor (15 GB per layer instead of 4.7 GB).
The HD DVD standard was jointly developed by Toshiba and NEC.
HD DVD has a single-layer capacity of 15 GB and a dual-layer capacity of 30 GB, the HD DVD-RAM has a single-layer capacity of 20 GB and a dual-layer capacity of Undecided.
Like the original DVD format, the data layer of an HD DVD disc is 0.6 mm below the surface physically protecting the data layer from damage.

-- From Wikipedia --