Perhaps this isn't what's being asked but without any further information...
Lets say we have a function box. We send a dry signal to that box. This box will split that signal into at least two (often times more). Half of this now split signal passes through dry, the other half is sent back and played at a later time (and almost always) at a lower volume. Sometimes the half of the signal that is sent back is split again...and again, and again each time it is sent back. This will achieve multiple delays that trail off until there is no volume. (Do note that some functions for delay make all the splits at once, so if you only want to hear the delay trail attack three times, you could make that happen).
The two important knobs on most delays are marked 'Regen' (How loud the delay trails are) and 'Delay' (how far apart the delays are in time).
If the delays are not in regular intervals, and are playing back at a variety of volumes, that's reverb.