http://en.wikipedia.org/wiki/S-VideoSeparate video, abbreviated S-Video and also known as Y/C is an analog video signal that carries the video data as two separate signals (brightness and color), unlike composite video which carries the entire set of signals in one signal line.
In something like out putting from a psi to tv the difference can be huge, ever when using tv-out the improvement is noticeable I have foundSvideo does have better quality than RCA, but don't expect anything amazing. Neither RCA or SVHS will look that great if your using tV-Out from a computer.
that is really not helpful, wiki whore