There a 3 types of Super Audio CD:
1. Single layer high-density SACD
2. Dual layer high-density SACD
3. Dual layer with 1 CD layer and 1 high density SACD layer
The Hybrid SACD is the third variety which has both a CD recording and a SACD recording on the same disc.
The CD layer can be played on normal CD players but the SACD layer requires a SACD player (or DVD player with SACD playback abilities).
For more on this check out the Technology link on this site:
http://www.superaudio-cd.com/
hth,
Owen