Voltage
Voltage is always measured between two points, and one of them is often called the "ground", or the zero volt (0V) point. In most AC electrical installations there is a connection to the earth. A connection is made to the real ground through a water pipe, a ground rod buried or driven into the earth, or a convenient metallic conductor (not a gas pipe) buried underground.  This connection is made at the point of entry of the electric system into a building, at every pole where there is a transformer at the street (often on an electric pole), and other places in the system. The whole planet Earth is used as a reference point for measuring voltage. In a building this ground is carried to each electrical device on two wires. One is the 'grounding conductor' (the green or bare wire) and is used as a safety ground to connect metal parts of equipment to the earth.  The other is used as one of the electric conductors in the circuits of the system and is called the 'neutral conductor'. This wire which is at the ground potential completes all the circuits by carrying the current from any electric equipment back to the systems entry point into the buildings and then to the transformer usually at the street.  In many places outside the buildings it becomes unnecessary to have a wire to complete the circuits and carry the current from the buildings to the generators.  The return path that carries all the current back is the earth itself.