I'm no expert either, but here's my understanding...
SACD and DVD-Audio are both high-resolution digital music formats. They use higher sampling rates to produce a more "faithful" reproduction of the sounds originally recorded.
Though the higher sampling rate undoubtedly produces a truer reproduction of the music than a redbook CD, it is questionable how much the technology benefits alone can actually be perceived by the common ear. However, most of the SACDs and DVD-As are remastered, and that can make a HUGE difference in the quality and enjoyment of the album.
I believe that many SACD releases are hi-res stereo remasters, with some titles having multi-channel surround sound. DVD-A releases tend to be the other way around (mostly surround-encoded with a few stereo-only). Correct me if I'm wrong on that.
Hope this helps!
