SCART is a definition of a connector, not of a signal.
SCART can carry:
1) RGB signals - Best quality you can get. DVD players, VGA cards, and arcade PCB's produce RGB signals. All colour signals are seperate AND the sync signal is seperate. Just like the guns in your TV/monitor wants them. The "only" thing the monitor has to do is get the color signals on the right level, and let the electronbeam be controlled by the sync.
2) S-Video
Lower quality than RGB and lower than component out. Carries all color info in one signal. The sync is the second signal. Because these are seperated, it's still beter than composite. However, the monitor has to seperate the three colors that are combined to be able to "feed" them to the elctron guns.
3) composite
All info on one signal, all colors AND the sync are mixed. Monitor/TV has to seperate the syncing from the colors and then the colors from eachother. It's the worst of the three. (Old) VCR's carry this signal, because this is the type of signal recorded on the tape. S-Video came later with S-VHS.
The SCART can actualy do much more than that, but this is the basics. There's plenty of info about SCART on the net....
but.....what actualy do you want to accomplish ?