Delay/echo units produce an echo effect by adding a duplicate instrument-to-amplifier electrical signal to the original signal at a slight time-delay. The effect can either be a single echo called a "slap" or "slapback," or multiple echos. A well-known use of delay is the lead guitar in the U2 song "Where the Streets Have No Name", and also the opening riff of "Welcome To The Jungle" by Guns N'Roses.
source: Wikipedia