The potential difference accross the terminals of a cell when it is not supplying any current is called e.m.f while
The voltage accross the terminals of a cell when it is supplying current to external resistance is known as Potential Difference .
When the cell is being charged
ε is, actually, a potential difference between two electrodes when current is not flowing.
P.D-potential difference is voltage across external resistance of load when current is flowing.
@Nilesh EMF, electromotive force, refers to the voltage developed by an electrical source. Potential difference refers to the observed difference in voltage between any two points in an open circuit.