I'm not sure who started throwing the term 'side-chain compression' around in the electronic music world recently, but it seems that every aspiring 'producer' (in the David Guetta sense of the word) under the age of 30 wants to know only two things; how much Ableton Live is, and how to set up side-chain compression.
Side-chain compression is a compression technique involving the use of an external signal to work the compressor instead of the signal it is applied to.
Why would you want this?
Well, maybe the kick drum and bass guitar are clashing in the low end, so by setting up side-chain compression, the bass signal is squashed by the compressor every time the kick drum comes through. This keeps the bass out of the way of the kick drum so that it can cut through the mix. It is also used when de-essing vocals, as the compressor can be set to work on a particular frequency. Another use for side-chain compressing is 'ducking'. Basically, a piece of music (usually on a TV program) is fed into the compressor with the voice over on the side-chain. When the voice comes into the picture, the music will 'duck' meaning that it drops in level.
You will have heard side-chain compression used as an effect in certain genres of music to make the track 'pump'. One More Time by Daft Punk is a fine example of side-chain compression being used to make a track 'pump' by lowering the level of everything but the kick drum. This process is also referred to as 'ducking'. I don't know any David Guetta songs to use as an example, and I'm thankful for that.
So, if your bass drum isn't punching through, or you want your tracks to 'pump', try side-chain compression.