Rackon's explaination is correct, I would add to that: Redbook is a format for 16 bits 44.1kHz sample rate. (That doesn't mean you can't over sample it--it's just that is the native format) Redbook is a single layer format (unlike SACD and DVDs) All standard CDs and CD players must be compatible and compliant to this. Some SACDs are dual layer and layer 1 is Redbook compatible--meaning you can play it back on a standard CD player. And as Rackon pointed out both SACD and DVD players will play back Redbook (standard) CDs.
