There are 2 types of SACD discs:
1. Standard SACD. Plays only on SACD players.
2. Hybrid SACD. The platter or disc itself has two layers: a standard Redbook CD and a buried SACD layer. This disc is playable on both CD and SACD players.
SACD, being a very high resolution format, sits on a DVD-type layer (4.7GB), but the format is proprietary and encrypted. After lengthy deliberations, a digital audio output format is finally ready, but found only in the high-end SACD players. Sony is particularly concerned about piracy, so it's doubtful whether they have any plans to make SACDs readable on a computer (i.e. SACD-capable drives) or even to license the digital audio output format for use by software/OS companies.