To a layman, EMF (electromotive force) is simply the voltage across a battery, and PD (potential difference) is the voltage across a resistor. Both of them are measured in volts (symbol V) by a voltmeter.
A crude but extremely useful analogy is that of a water pump (battery) that raises water (electric charges) to a tall height (high electric potential), whereupon the water (electric charges) is allowed to fall through a water mill (resistor) to return to ground level (ground potential). So the EMF is the potential difference the charges are raised by the battery, and PD is the potential difference the charges drops through the resistor. Since the charges are always returned to the ground potential after completing the circuit loop, the EMF of the battery is equal to the PD across the resistor. If there are more than one battery or resistor, then the total EMF of the circuit is always equal to the total PD of the circuit.
So that’s the layman’s idea of EMF and PD. Now let’s hear what the physicists have to say about these.