Applications
Voltage is a differential quantity, which appears between two points. In order to deal only with a voltage (an electrical potential) of a single point, the second point has to be connected to a reference point (ground). Usually, the power supply terminals serve as steady grounds; when the internal points of compound power sources are accessible, they can also serve as real grounds.
If there are not accessible source internal points, external circuit points having steady voltage with respect to the source terminals can serve as artificial virtual grounds. Such a point has to have steady potential, which does not vary when a load is attached.
Read more about this topic: Virtual Ground