Composite video is a video signal in which the luminance (brightness) and chrominance (colour) parts are combined into a single signal. This is in contrast to component video, in which different parts are carried separately.
Composite video has the advantages of being convenient and using less bandwidth. However the quality of the signal is compromised; in particular, the colour tends to "bleed" and become less-defined. Once luminance and chrominance signals are combined, they cannot be cleanly separated again, so the quality loss in composite video cannot be compensated for.
Being an analog system, composite video has been superceded by various digital signals.