The S-video connectors/cables carry video using two synchronized signals, Y and C, and a ground pair.
Y is the luma signal that carries the brightness or black and white of the picture, including the sync pulses.
C is the color signal that carries the chromaticity – or coloring – of the picture. This signal contains two color difference components.
The luminance signal carries horizontal and vertical sync pulses in the same way as the composite video signal.
In composite video, signals coexist at different frequencies. To achieve this, the luminance signal must be low-pass filtered to dull the image. Because S-Video shields the two as separate signals, such harmful low-pass filtering for brightness is unnecessary, although the chrominance signal still has limited bandwidth relative to component video.