VCA stands for 'Voltage Controlled Amplifier'. These appear on large, expensive mixers and are extensively used in large musical productions.
They appear on the mixer as a numbered fader, and beside each input there is a control for setting which VCA that input is assigned to. (it can also, of course, be assigned to no VCA).
Thus, you can assign any number of inputs to a VCA fader, and this VCA fader will then control the level of that input - remember, however, that the fader for each channel will have to be raised - usually to 0dB or the unity gain position.
You can therefore control the level of a group of related inputs - commonly in a musical you will have VCAs assigned with the band inputs, radio mic inputs, float mics and replay inputs as well as perhaps effects returns.
The advantage of using VCAs over groups is that the groups are often used for their outputs, and assigning channels to a group means that you will send those inputs to that output, whereas by using the VCA, the inputs can still be assigned to any outputs and be grouped together.
If you are using the auxilliary outputs, you must check how the VCA affects the Aux sends from each input channel. With some Sound Desks it is possible to configure the Aux sends, so that they are not affected. Therefore it would be possible for a signal to be live via the aux send even if the VCA fader was right down.