what is "redbook"?

I have been reading some forums here recently and have seen people writing about 'redbook cds'-Can anyone tell me exactly what that is?
"redbook" CD's are "standard everyday CD's". This term is used to differentiate it from the "specialty" CD's like Super Audio CD's ( SACD ), CD-R / CD-RW's, Video CD's, etc... Due to having different formats, standards and technical aspects, it became necessary to seperate and categorize the different types of CD's for sake of clarity. The "redbook" is the set of specifications that lists the standards for what we think of as the "standard" CD. Sean
To expand on what Sean said with some basics: Red Book CDs are 2-channel only, sampled at 44.1 kHz and use 16 bit word lengths. Some of the recent variations are multi-channel recordings, sampling rates of 48 kHz, and 96 kHz, and word lengths of up to 24 bits for PCM encoded data. For SACD, multi-channel is also possible, the sample rate is 2.811 MHz (I think) and the word length is 1 bit.
Thanks to both of you
Thank you as well. We really need an audio glossary on this site :-).