Reverb is the extension of a sound after its source is interrupted. It is caused by the multiple and tight reflections of the sound on the different reflective walls of the space where it projected. It creates an effect of diffuse sound extension, longer or shorter depending on the size of the place and the reflective properties of its walls. It creates a psychological effect of power, depth, mystery.
Not to be confused with the echo. The echo phenomenon consists of one or more reflections of the original sound that are sufficiently separated from one another in time to be perceived as repetitions of the initial sound.
Example: 1. dry sound 2. sound with multiple echoes 3. sound with reverb