What is side-chaining?

I see side-chaining on VST comps and other effects like delays. I saw a video on you tube where a guy set up side chaining on delay.
In that video the delay was only active when no signal was present on the track. In between the notes-so to speak.
Is that all side chain is? What is it basically? I see it all the time and wonder if I should be utilizing it.
Thanks alot for any responses.

Side chain allows you to use an effect on 1 track while having another track control the effect.
For instance a compressor:
Normally, a compressor would take a signal at its input, and if that signal exceeds a certain limit (the threshold) it will apply gain reduction. When using a sidechain, you can apply the gain reduction to a track not because of the signal levels of that track, but of another track. So if you have a pad sound going into the compressor and a drumtrack going into the sidechain, you get gain reduction on the pad sound whenever a drumsound plays.

In short you open up the detection part of an effect to another signal than the one you’re processing.

Here’s a compression view of side-chaining: http://en.wikipedia.org/wiki/Dynamic_range_compression#Side-chaining

In the Cubase 7 manual, it is discussed starting on page 240.

Generally speaking, Side-chaining is placing an effect on one track and using another track to determine when that effect is applied (or not applied). It’s certainly capable of being more complicated than that, but that’s a simple version.

Great replies and thank you all.
It is complicated. What would one use that for? Any typical reason?
Like I said, I saw the guy on You tube use it to take away the delay “wet” signal while the actual guitar
signal was active. So the delay wet signal was only heard in between the notes. It kind of cleared it up and made his riffs less muddy. That was a good technique.
What else could you do with it? Especially in the example Strophoid mentioned?
What is the purpose of using another track to effect the comp on another track?
Thanks alot. I appreciate the patience with a Mega-Newb.

Using a compressor with a sidechain allows you to ‘duck’. It’s very clearly noticeable in EDM music where they sidechain the kickdrum into backing synths so you get this pumping sound of the kickdrum and synths alternating. This allows you to make the kickdrum louder than you usually could because there’s no other sounds playing together with it. The wikipedia link from Chris has an example.

+1
Watch/listen to a sporting event on television.
When the announcer speaks, the background crowd noise goes down (ducks).
When the announcer stops talking, the crowd noise rises.
In American football (and many other sports) this gives a dramatic effect just before the ball is ‘snapped’.

{’-’}

Thanks Curteye and Stroph. I’ll keep this info in the back of my head. It might come in handy.
As of now I’m just getting the hang of basic mixing and EQ down.