Expanding on what royphil said, redbook or orangebook CDs are the LEGACY format based on stereo PCM at 44.1kHz. Too much emphasis was put on the Nyquist theorem as being adequate for subtleties in the upper spectrum of music. Consequently, SACD and DVD-A mean to surpass Nyquist by a wider margin, giving more sample points to upper octaves. Higher bit rates help, but noise shaping did a lot for linearity of regular old redbook.

Here comes the bad part. There are MATHEMATICAL CONVERSIONS of older recordings coming out on SACD. This is like HDTV programming that is actually upconverted regular programming. It might be the new format, but with the weaknesses of the older recording technology added to whatever new format weaknesses there might be. SACD recordings to buy would be new ones, mastered with the technology, and not upsampled. The Technics A-10 version of the Panasonic DVD-A player, has upconversion as optionally activated circuitry to play redbook CDs through the DVD-A digital chain. Reviews were mixed, and similar results would be expected for upconverted SACD.