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You can think of delay as being an echo or repeat of your guitar sound. Delay is used in everything from early rock'n'roll to the most modern rock and pop. If you've heard a U2 record you've heard delay, but there are many more applications for this effect.
Most delay pedals will feature a level control that effects the volume of your repeats, a feedback control that effects the number of repeats, a time control that effects the interval between repeats, and some models include an additional control that allows you to line up different flavours of delay.