Sweep picking is almost like strumming a chord, but instead of strumming the chord, each note is picked individually. The "strumming" hand sweeps the strings with a pick while the fretting hand fingers and mutes each seperate note. Steve Vai, John Petrucci, Yngwie, etc....use this technique a lot. It's hard to explain....

An example would be Steve Vai's "Eugene's Trick Bag" from the movie Crossroads (not the Britney one).....