Clap your hands! Go on, don't be shy…
What you just heard wasn't only the sound from your palms hitting together. You also heard sound reflections from the walls and surfaces of the room around you. Try the same experiment wherever you go, at least where it doesn't cause too much embarrassment, and you will hear that every room, hall or auditorium sounds different. Even outdoor spaces sound different. The reflections from a handclap sound very different among woodland than in a narrow alleyway between buildings.
We call these reflections 'reverberation'. Reverberation is present wherever there is sound. If you record a skilled acoustic instrumentalist in a good acoustic environment, you will try to capture the natural reverberation as accurately as you can.
But in the studio, it isn't possible to acoustically simulate every acoustic environment. So it is more common to try and record as 'dry' a sound as possible, then add reverberation to it digitally.
Digital reverberation plug-ins attempt to simulate natural reverb. They also simulate artificial reverberation devices from the past, such as plate reverb units. They can also create reverberation effects that are quite different from anything you would hear in real life.
Reverberation can be used to simulate a natural acoustic. Or it can be used to fill out a mix. If the instrumentation of a recording is thin, or the recorded sounds themselves are lacking in fullness, digital reverberation can be added to compensate.
We would talk about reverberation as being an 'effect' used in recording. There are other effects too. A simpler form of reverberation is delay, where there is just one single reflection, and 'spin echo', where the reflection repeats at regular intervals and dies away over a time.
There are effects that can 'thicken up' a recording, such as phasing, flanging and chorusing. These are subtly different to each other, but the basic idea is that a cyclic pitch change is applied to the signal, and mixed back in with the un-effected signal.
Pitch change can be used as an effect. It is commonly used in science-fiction drama to create robot voices. Pitch correction software can be used to retune an out-of-tune vocal. Or it can be used as an effect in its own right.
When and why to use effects
Reverberation is used to emulate something that happens in real life. Other effects are used either to thicken up signals, or add 'ear candy' to a mix. Often effects are added to compensate for something that is lacking in an original recording. Effects however can sometimes be an intrinsic part of a sound – an electric guitarist will commonly have a pedal board, fitted out with several effects units with which he can modify his sound. All of these effects can be emulated in the studio using digital plug-ins.