I have a G&L Tribute guitar (made by Fender, but not quite as good as a strat) and it buzzes loudly and constanly when i play....my friends tell me because it's single coil, but what does that mean?
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All single coil pickups have buzz. I beleive it is called 60-cycle hum. Buy switching your switch into the 2nd or 4th position you can cancel out the buzz. By activating 2 of the pickups at the same time the buzz cancels eachother out.

You can always buy a noise gate pedal also. But they seriously take away from your tone a lot.
Listen to some SRV. That's single coils. You probably have fretbuzz, or defective cord and/or jack. Strings are supposed to vibrate freely. If they touch the frets while vibrating, a buzz sound will appear. Strings set too low on the fretboard is one cause. Raise action.