Whenever an
event is either recorded (audio) or filmed (
video or
film), post-processing is the
process by which the material is
edited,
enhanced, and otherwise modified so that the
result is closer to what the
producer imagined.
Some examples of post-processing are wipes (video), dissolves (video and film), QSound (audio), mixing (usually audio, but sometimes video editing is referred to as mixing as well), equalizing (audio), color correcting (video and film), colorization (black and white film or video; some people hate this }:) ), and so on.
It would probably take several megabytes to explain all these different types of post-processing; I have left them as hard-links so any future noders who are in these fields can add their knowledge.