S-Video (Y/C Y=luminance C=Chroma) provides a better quality signal than composite (cV composite video) however, cV is still a universal cross between video systems including DJ gear, security monitors, industrial and consumer electronics.
Y/C can be run up to about 150ft without any kind of repeater or amplifier. How well it works after that depends on how many devices you are feeding and whether the signal is being split. cV will run well at twice that length, but should have a repeater for exceptionally long lengths.
You can transmit even farther by using RF instead of line signals, or a balun to cat-5 will let you run 1000ft.
cV is considerably less expensive for monitors, switchers, distributoin amps etc. and that's why you find it on most DJ gear. The RCA connectors are used on consumer gear and the locking BNC is the industrial standard. BNCs last longer, are a locking connector, and provide better termination.
All analog video is slowly being phased out and the first to disappear will be S-video. You have probably already noticed that it is no longer supported on computers, and many DVD players, cameras, or TVs.
You're best not to get too deep into S-video since most devices you acquire in the future will not have that input/output. Consumer S-video is also a fragile 4-pin connector that can easily be broken. Industrial S-Video uses a pair of separate BNC connectors (Y/C)
You can passively convert a Y/C signal to cV however, there can be only one such source on the branch. Mixing a passively converted Y/C signal with another S-video source through an active switcher will cancel out the chroma information on one or more outputs.
To convert from YC to cV or vice versa you should use an active combiner or separater to insure that all your gear in the chain processes the signals correctly.
For karaoke display or music videos S-video or composite is all you need.
VGA, (SVGA, XGA, ...etc.) are all data display modes and are analog component variants (RGBHV, RBGS, ...etc.) VGA ports may all look the same (DB15 connector) - but they are not! You must know exactly what type of video synchronization scheme is employed by each device. If your devices do not allow you to select a sync mode you must select devices using a compatible standard.
If you are going to display fine text that must be read by a large audience such as spreadsheets, powerpoint, etc. VGA is the better choice. VGA images will be brighter than comparable content in S-video or Composite. It's also readily scalable.
The maximum run on VGA is about 100ft directly out of a PC. VGA amplifier/splitters have to be within 6ft of the PC and then you can make longer runs out to your displays. Passively splitting a VGA signal will not always leave you with enough signal strength to drive all the displays.
Because VGA is an analog format it too is being phased out. You'll notice most consumer PCs are now outfitted with HDMI. Business machines will have the VGA port for a while longer because of the installed inventory of analog data projectors in institutional settings.
Expect HDMI to be the standard for the future. It can carry 8 channels of audio plus control information in addition to HD video.
