There is a distinct difference between emf and potential difference. The Emf of a device, say a battery, is a measure of the energy the battery gives to each coulomb of charge. Thus if a battery supplies 4 joules of energy per coulomb, we say that it has an emf of 4 Volt. The energy given to the each coulomb in a battery is due to the chemical action.
The potential difference between two points, A and B, is a measure of the energy used by one coulomb in moving from A to B. Thus if potential difference between point A and B is 2 Volts, it means that each coulomb will give up an energy of 2 joules in moving from A to B.
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