What is a virtual ground? I would like to know what it is.
Virtual ground refers to a circuit element not directly connected to ground, held at a reference voltage. This reference voltage need not be the same voltage as ground either.
For example many op-amp circuits were originally designed for dual power supplies (say +12V and -12V) and could handle filtering or modification of a signal that was oscillating about ground. This need for two power supplies is annoying, so it is possible to remake these circuits with a single supply, and a "virtual ground" held at half the power supply. As long as signal voltage are compared to this virtual ground, filtering or modification of the oscillating signals can be handled just fine as they only go negative relative to the virtual ground.
In this way, it is fairly straight forward to convert dual supply op-amp circuits with single supply circuits by replacing the ground reference voltage in the circuit to a virtual ground to which the signal is referenced. This is the main context in which I have seen 'virtual ground' have a useful application.
"Ground" refers to a particular voltage, generally taken to be "zero", or the voltage of the earth. A "virtual ground" is a wire in a circuit whose voltage is held to be zero not because it is directly connected to the true ground, but instead because it is actively driven to that voltage typically by feedback mechanisms.
Here's an example of a virtual ground circuit:
In the above, the large triangle represents an OP-AMP, that is, an operational amplifier. It drives its output (marked VGND) to be + voltage when its + input has a higher voltage than its - input, and to be - voltage when its - input has a higher voltage than its + input. Since the + input is connected to true ground (marked GND), the op-amp will keep it output VGND at the GND level (to the best of its ability). Since the VGND wire is held at ground but is not connected to ground, it is called a "virtual ground".
By the way, in the usual "electric-circuits" limit, circuits are composed of "wires" where we make the assumption that the voltage of a wire is the same no matter where on the wire we look. That is, we ignore the process by which a signal is sent down a wire.