Superbit DVDs are DVDs that use all the space for the just the movie and the soundtrack - no extras such as commentaries or other special features are included on the same disc. The reason for this is that the Superbit process uses the entire bit-rate (thus the name Superbit) capability of a DVD disc, thus maximizing the quality of the DVD format. As a result, the colors have more depth and variation and there is less edge artifact and video noise issues. Think of it as an "enhanced DVD".
However, although Superbit DVDs provide an improvement in image quality over standard DVDs, they are still not as good as a Blu-ray or HD-DVD disc. Superbit DVDs are still 480i resolution. On the the other hand, if you have a progressive scan DVD player you can output a Superbit DVD in 480p and you can also do upscaling up to 1080p when played on an upscaling DVD player.
Superbit DVDs are playable on all DVD and Blu-ray Disc players. However, since the introduction of Blu-ray, Superbit DVDs are no longer being released.
For more details on Superbit DVD, refer to the following article: A Look at Superbit (DVD Talk). Also, check out a list of all Superbit DVD titles that were released (note that the Now Available link is no longer active) as well as a very good visual comparison between Standard DVD and Superbit DVD.
