SACD -- Hybrid, Single layer, Stereo, Multi?

Rar1 --

Thanks for the link. What I am wondering is -- whether
or not a Stereo SACD usually includes a multi-channel
SACD version and whether a multi-channel SACD usually
includes a Stereo SACD version -- AND -- if there is
a qualitative difference between the play-back of a hybrid disc and a pure DSD disc.

Mostly, I would like a definition of these terms so I
have some idea what I am getting when I buy a SACD, but
I also want to know if having a regular CD layer in any
way reduces the quality of a SACD.

Kr4 and Aroc -- Thanks for the info!