Time Delay   

Time delay effects are one of the most commonly used techniques in audio. 
This type of processing involves generating a copy of a portion of the signal and playing it back at some time interval later.  In its most basic form the time delay effect manifests itself as a simple echo. 
This can be elaborated on by feeding an attenuated portion of the delayed signal back into the delay line which will result in a series of decaying echoes.  The time between the original signal and the delayed signal can be modulated to create more variation in the effect.  Stereo delay effects can alternate repeats between left and right channels or pan the repeats at variable rates. 
See Haas effect.
Depending on the length of the delay an number of different effects can be achieved, such as echo, doubling, chorusing, and flanging.