Redbook is the formal CD standard- it states all the parameters required to make an audio CD. There are other standards for computer use (Orangebook and, I think, at least one more). This applies to CD-Rs, too- that is to say, to play back as a CD they must conform to redbook specs.
Interestingly, if you change that spec you no longer have a CD and you can't market it as one. Philips has sent cease & desist letters to companies in the past who's copy protection schemes have nudges the format beyond the boundries of the Redbook standard. This is important in the sense that anything labelled as a CD should be 100% compatible with all CD players.