This document covers audio delay, echo, reverberation and phase inversion.
Delay
Delay is simply delaying the audio by a fixed amount. When a very small delay, between about 5 thousandths of a second (5 milliseconds) and about 50 thousandths of a second (50 msec) is applied to one mono channel as opposed to the other, the human ear will be tricked into creating a stereo sound field where different sounds appear to spread across the space between the left and right ears. This is because the delay affects higher frequencies more than lower frequencies and is why it is easier to pinpoint the direction of a higher frequency as opposed to a lower frequency. If the delay is too long, the ear is able to discern an echo. Echo becomes apparent when the delay reaches about 1 tenth of a second (100 msec).
Echo
An echo is a delayed replica of the audio that is mixed in with the original audio. An example in nature, could be a shout reflected off the wall of a canyon. There can also be further reflections that die out over time until no longer apparent. We call this an echo because the delay is long enough for us to discern two separate sounds. When the audio is continuous, such as speech or music, we can still discern the separate sounds.
Reverberation
Reverberation is really the same as echo except that the delay is so short that we can't clearly discern separate sounds. This occurs when the delay is shorter than about 100 msec and gives rise to a perception of ambience or "liveness" in the audio. The shorter delay provides more opportunity for the delayed audio to influence and interact with the original sound. Reverberation is apparent in enclosed spaces like concert halls or stadiums or even small halls and is affected by the shape and treatments on the walls like curtains and other things like furnishings and even people, all of which can dampen or alter the reflection of sound. Enclosed spaces and their affect on reverberation can impose a lot complexity on the character of the overall sound.
Phase Inversion
Phase inversion is inversion of the sound wave as compared to the original. In electrical terms, this means that where the original waveform is increasing in amplitude, the inverted waveform is decreasing, and vice versa. In sound terms, it means that where the sound pressure goes in one direction in the original, it goes in the other direction in the inverted version. If you could put your hand on two separate speakers with one producing the original sound and the other producing identical but inverted sound, at any given time, one speaker would be moving in and the other would be moving out. For the ears, this has the affect of moving the apparent position of the sound. In-phase mono audio sounds centred but the position of out of phase audio is vague and uncertain.