S-Video is a video format that provides a higher quality picture than standard composite video but a lower quality picture than component video. The standard 4-pin S-Video connector looks like this. The main advantage that S-Video has over composite video is that while the latter carries all picture properties on one signal, the former separates color and brightness, which allows more information to be transmitted from the output device to the input device; this grants the viewer a better and more distinct picture than would be available with standard composite cables. S-Video, appropriately enough, stands for "separate video," which neatly describes the premise behind it. Like composite and component video, S-Video has no audio capabilities, so separate connectors are necessary if you actually want to hear what's going on.
The idea for S-Video has been around since the 1970s, finding usage mainly in the early home computing world. In 1987, the Japanese electronics company JVC conceived of the Super VHS format as an answer to consumer demands for cassettes that provided better picture quality than standard VHS tapes or the recently discarded Betamax format. JVC took the separate video format and standardized it to include 4 pins instead of 7 or 9 or any other number that could be found on various proprietary cables. The picture quality was noticeably better; the standard resolution for VHS was 330x480 while S-VHS was capable of 560x480, comparable to the Laserdisc format that was then held in awe by the average consumer. Due to its association with Super VHS, S-Video is often incorrectly said to stand for "Super Video." Unfortunately, S-VHS never really took off the way that JVC hoped it would. For one thing, S-VHS tapes and S-VCRs were significantly more expensive than standard VHS tapes and VCRs, and most televisions did not support the S-Video format, which was essential to getting the full effect of an S-VHS tape. It would be like watching a DVD by using an RF cable: you can do it, and it looks a little bit better, but why would you? Also, many companies were unwilling to invest in an untested format; the average person wants things to be better, but they'll be damned if they're going to pay for that quality.
The main market for S-VHS and by extension S-Video was the luxury electronics consumer, although at that point, those people were buying Laserdiscs instead of S-VHS tapes anyway. Like Betamax, it found some "posthumous" success in the world of television production, but not too much. With time, however, S-Video inputs became standard for big screen TVs, even if a lot of people weren't using them. As production costs and therefore prices declined, more people found themselves with the ability to utilize S-Video if they so desired. With the appearance of the fifth generation video game consoles and the DVD format in the 1990s, S-Video finally found a real market. An S-Video cable was not typically included as a standard piece with any piece of equipment at this point, but it became reasonably popular since it still provided a better picture than either RF or composite video. Most aftermarket multiple-use video cables include an S-Video attachment to this day.
Today, the RGB component cable is the standard for separate-channel video. It provides a significantly higher quality picture than either S-Video or composite video, although it is backward compatible with both of them. High Definition TVs have only in the last couple of years started to become affordable investments for the average electronics buyer, but there are still many people who find them just out of reach. S-Video is still an attractive alternative for these types of people as well as for those who have waaaaaaaaaay too many things plugged into their TVs and need just one more plug to get everything in there.