JC did a great job, but I wanted to offer a little history and zero in on the term "VCA" and what it really refers to. What we have in Pro Tools is really a "virtual VCA master" and here's why. In an analog console(with VCA's) every channel has a VCA(Voltage Controlled Amplifier) that raises and lowers the volume of that channel. Each VCA is controlled by the channel's fader because the fader controls a voltage(0-10vDC) that feeds the VCA(so the fader actually passes no audio, only DC voltage). The VCA Master faders simply over-ride the voltage of the channel VCA's that are assigned to that master.
