These discs have the capacity to hold up to 3.9 TB of information, which is approximately 6,000 times the capacity of a CD-ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-Ray discs and about 8 times the capacity of standard computer hard drives as of 2006. The HVD also has a transfer rate of 1 gigabit/s. A 3.9 TB HVD could hold between 4,600–11,900 hours of video—just over one year of uninterrupted video.

Though still in the research stage, it employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. However, Maxell is expected to release in September 2006 a disc with a capacity of 300 GB and transfer rate of 20 MB/sec.

For more info – http://en.wikipedia.org/wiki/Holographic_Versatile_Disc