An abbreviation of Separate Video and marked or referred to as Y/C. Carries and 480 or 576 interlaced signal separated into Luminance (Y)
and Chrominance (C)
The luminance part of the signal is the image in greyscale (which is why we can sometimes see a black and white image with SCART to S-Video cables) while the chrominance signal carries the colour information.
S-Video will give a noticeably better image than composite but its quality does not match RGB, component or digital signals.