What is meant by the term sweep picking is when the guitarist picks the string downwards and then the next string below it downwards its called a 'Sweep'. This also applies for upward strokes. Guitarists usually sweep pick arpeggios, as they get a specific neo-classical sound to their music. You can sweep pick scales aswell althuogh this requires the use of the legato tecnique combined with the sweep picking. Also you can sweep pick and add a tap on the end of it, this is known as a Hy-peggio.