SACD is Super Audio Compact Disc. It uses a disc similar to the DVD in that it can be multi layered and multi sided. It can store up to 17 Gigabytes of information as compared to a CD which stores only 640 Megabytes. It also uses a completely different encoding scheme refered to as DSD. This scheme differs from traditional PCM (used on CDs and DVDs) in that it uses only one bit at very high sampling rates (something like 2.4 MegaHertz as opposed to the 44.1 KiloHertz of CD). Sony was one of the initiators of this technology and has an area on their web site dedicated to the explaination of the technology. It is:
http://www.sel.sony.com/SEL/consumer/sacd/flash/index.html
http://www.sel.sony.com/SEL/consumer/sacd/flash/index.html