In an electrical circuit there are always points with different electrical potential. For convenience when studying an electrical system is considered a point in the system as a reference, for the same reason for which to measure the altitude of the earth’s crust is considered the sea level as the reference altitude. Below you can see the symbols to indicate the grounding:
This particular point of the electrical network is called ground. The electrical potentials of all the other points of the network are evaluated with respect to the ground potential.
Another point of reference widely used in the electrical sector is earth, to which the metal parts of electrical equipment are generally connected in order to protect users who work with them.
The electric potential to which the earth is found is by convention null. Therefore the mass of an electrical circuit can be at a different potential than that of the earth, and therefore from zero.