Echo is caused by the reflection of sound from a large reflecting surface or obstacle.
An echo can be heard only if there's a minimum distance of 17.2 meters between the obstacle and the listener. An echo is mostly clear and distinct.
How ever, a Reverberation is caused by caused by the over lapping of many echos. Reverberation is the term used for persistance of sound due to super posing of many echos. A reverberation is often muffled and unclear.