What does it do?
Delay is one of the simplest effects and, in fact, serves as the basis for many other more complex effects.
A delay processor does exactly as the name suggests: it delays the incoming signal, sending it out later than it came in. Sometimes it’s used as a corrective tool - more often, however, it serves as a sound-enhancing effect.
When combined with the un-delayed signal, it creates an echo effect. Sometimes many copies of the echo will be present, and (usually) each will be successively lower in volume than the one preceding it. You might hear this effect on, say, the last line of a vocal, or on a guitar line (think U2 or David Gilmour).
How does it work?
A delay works by copying the signal and then playing the copy back after a user-specified period of time.
Old delays were made by using one or more tape machines and changing the distance between the record and playback heads. Digital versions abound, since playing back a copy is not a difficult task for modern computers and software. You will often get control over the delay time, feedback (the number of echoes) and mix between the original and delayed signals.
Sometimes there will be some control over the frequency content of the echoes; this simulates the interesting and characterful degradation inherent in old tape-based units.
Use it on...
Short delays sound terrific on vocals and were often used to treat the vocals on John Lennon and Buddy Holly records, to give but two examples. They also sound great on rhythm and lead guitars, and on synthesiser passages.