an ebow is a hand held battery powered electronic device which stimulates vibrations in a guitar string without having to pick or pluck the string,so you have almost no attack and unlimited sustain. Check out old Bebop Deluxe records to hear Bill Nelson wailing away on ebow guitar!
p.s. I once heard about,but did not hear, a project where some pianist rigged up an array of multiple ebow on a grand piano and rigged the keys to not strike the strings---just to lift the pads and let the ebow vibrate the strings. I bet he got some cool sounds out of that!