A voltage must always be measured relative to some reference point. It is proper to speak of the voltage across an electrical component but we often speak of voltage at a point. It is then assumed that the reference voltage point is ground.
Under strict definition, ground is the body of the earth. It is an infinite electrical sink. It can accept or supply any reasonable amount of charge without changing its electrical characteristics.
It is common, but not always necessary, to connect some part of the circuit to earth or ground, which is taken, for convenience and by convention, to be at zero volts. Frequently, a common (or reference) connection of the metal chassis of the instrument suffices. Sometimes there is a common reference voltage that is not at 0 V. Figure 1.2 show some common ways of depicting grounds on a circuit diagram.
Figure 1.2: Some grounding circuit diagram symbols: a) earth ground, b) chassis ground and c) common.
When neither a ground nor any other voltage reference is shown explicitly on a schematic diagram, it is useful for purposes of discussion to adopt the convention that the bottom line on a circuit is at zero potential.