Faders control the level of audio passing through the output of a channel. Most faders have a detented resting position about 2/3rds up its path. This is usually called 0 or “Unity” (sometimes signified by a “U”). This represents the position where no amplification or attenuation is done to the signal, or in other words the signal is neither turned up or down. As soon as the fader is raised above unity gain amplification is applied to the signal in order to make it louder.If the fader is brought below unity the signal is attenuated, turned down, or resistance is applied to it.
Virtual Faders are usually found in a software mixer, control surface, or digital mixing board. Virtual faders control audio passing through a virtual mixer. They are not specifically connected to an amplifier or attenuator like the faders mentioned above. They simply send and receive commands of the location of the fader in its path. For example, if a virtual fader is moved up, gain is added to the digital audio passing through the virtual mixer through mathematical calculations done with the digital code of the audio. If the fader is brought down calculations are done to attenuate the digital audio signal. There are no physical devices connected to the virtual fader because it is simply more like a plus or minus button on a calculator than a fader. It just looks like a fader in order to be familiar and consistent to other aspects of the audio recording world.
Voltage Control Amplifiers are different than standard faders because they do not pass an audio signal through them. They can be assigned to control other faders, groups or function that do pass audio. VCAs are commonly used to control all of the faders of a group simultaneously on analog consoles and in most DAW programs, but are also seen assignable to a myriad of functions in analog and software synthesizers. Many digital consoles and DAWs have changed the name VCA to DCA (Digitally Controlled Amplifier) to be more accurate to the digital world (where there are no voltages being controlled), but of course they serve the same purpose as VCAs.
There are other types of faders that you may hear about like moving faders, flying faders or motorized faders. These are all similar in that they are physical faders on an analog or digital console that can be moved by motors into a specific position.