S-Video is a connection commonly found on DVD, S-VHS VCR, Digital Camcorders (Hi-8 / Digital-8 / DV), and other associated video equipment.   The most common connection found on video equipment is the 4-Pin S-Video connection.

S-Video is short for  'Super-VIDEO'.   This cable standard was introduced for two reasons;

To reduce crosstalk between luminance and chrominance signals

To increase the luminance bandwidth capability of the TV by eliminating the need for a low pass filter in the luminance path.

The S-Video cable works by transmitting video signals only over a cable by dividing the video information into two separate signals;  One for colour (commonly referred to as Chrominance), and the other for brightness (referred to as Luminance).  The two signals are usually carried upon two individual coaxial cables rather than two separate wires in a bundle.   As the signals are never in a composite state, cross-luminance dot crawl on the vertical edges of a highly coloured area don't appear and cross-colour strobing doesn't show up (ie like on tweed jackets for instance).  There is also no longer a need for a  low-pass filter in the luminance chain inside the TV to remove the chrominance signal, so increasing the horizontal resolution (bandwidth)   When the S-Video signal is sent to a Television, this produces noticeably sharper images and more clearly defined colours than SCART or Composite video connections sent over an individual wire.  

One of the reasons why S-Video works so well is that TV display tubes are designed to display separate luminance (Y) and chrominance (C) signals.

Don't forget of course, when using S-Video connections with your DVD and TV, you must also provide a separate Audio signal.

BTW, the S-Video connector is a 4-Pin miniature-DIN.   The same connection is used on Apple Macintosh Keyboards.  You could use an S-Video lead to extend your Mac keyboard but unfortunately not vice-versa as this requires some electronics.