The job of a battery in a circuit is to convert stored chemical energy into electrical energy which drives charge to flow through the circuit. This energy is then used by elements in a circuit to do specific tasks. Each of the tasks performed by the circuit, whether it is simple like generating light or complex like storing information or making decisions within a digital chip, uses up some of that energy. In order to know exactly how much energy there is in a circuit requires you to know exactly how much charge there is. This can be difficult. Instead of measuring the energy itself, scientists measure the electric potential of a point in the circuit. That is how much energy one coulomb of charge would have at that point in a circuit. Any parts of a circuit that are directly connected by a wire have the same electric potential. Any parts of a circuit that are separated by a battery or by a circuit element with any resistance would have different electric potentials. This allows us to measure the potential difference, or voltage between any two points in a circuit.