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You will find side chain inputs on compressors, limiters, expanders and gates. But you won't find them on EQ units. Now there's a good question to start off with... why no side chain input on an EQ?
An equalizer is used to control the frequency balance of a signal. There are controls for high frequencies, mid frequencies and low frequencies. The engineer operates the controls, and the settings he or she selects affect every aspect of the signal - every frequency and every level. Whatever the signal is, the same EQ characteristics will be applied.
A compressor is used to control the range of levels of a signal. To do this, it must measure the changing level of the signal, and reduce the level of the loud sections, then boost up everything by a set amount so that the overall peak level is the same.
Clearly, the engineer needs to have control over what the compressor is doing. But the compressor is to some extent autonomous. It measures the signal level from moment to moment, and by itself decides how much gain reduction to apply.
So the compressor bases its operation on the characteristics of the signal, while an equalizer processes everything that goes through it in the same way.
Now for a leap of thought - if the compressor bases its operation on the characteristics of the signal, might it not be possible for it to base its operation on a different signal - different to the one it is processing? The answer is yes, and this signal is called the side chain signal. Clearly, because an EQ doesn't measure the signal in any way, nor use it to control its operation, an equalizer has no side chain, and therefore no side chain signal input.
The classic use of the side chain of a compressor is de-essing. But let's skip over to the noise gate and give an example of how the side chain can be used there...
In standard operation, the noise gate measures the input signal level. If the level goes below a certain threshold, the gate assumes that all it contains is noise, and therefore closes it off. When the signal goes above that threshold, the gate assumes that it is a wanted signal, and therefore opens fully. But the opening and closing of the gate can be controlled by a completely separate signal. This is sometimes called the external key, or external trigger, but it is just another form of side chain.
Suppose you are recording a drum set and want to gate each drum so that its channel only opens when the drum is actually being played. This results in a very much cleaner drum sound.
But you have a problem... the gate on the snare drum opens every time the drummer plays the hihat, and there is no setting of the controls that you can find that prevents this. This is a very typical scenario.
What you can do is tape a contact mic to the shell of the snare drum, and feed this - through a preamp of course - to the side chain (external key) input of the gate. Switch the gate to trigger from the external key.
Now, because the contact mic is unaffected by sounds passing through the air, it provides an ultra-reliable trigger for the gate. The gate always opens promptly, and never in response to anything other than a snare drum hit.
The side chains of compressors, limiters, expanders and gates have many other uses. Learning these is all part of learning how to be a good sound engineer.