alternate picking is strict 'down-up-down-up' motion regardless of movement between strings.

Sweep picking is more like strumming a chord. A single downward or upward motion crossing strings in the same direction and fretting notes as appropriate. You mute with the palm of the picking hand, and take your fingers off the frets right after you hit the note, so you get each one sounding separate, rather than the notes blending in to each other.

Sweep picking and economy picking are strictly speaking the same thing - it's just about whether you stick to strcit 'down-up-down-up' (hence 'alternate' picking), or whether you move the pick in the direction of the string you are moving to ('economy' comes from the minimal movement needed). Sweep picking is usually used to blast through an arpeggio ultra-fast...

Try a forum search - there's a fair bit on it. :cool: