Emf is the electromotive force. It is the potential difference across the terminals (anode and cathode) of a battery. EMF is the potential maintained by a battery and is constant during the use of the battery. It is independent of current in the circuit. Potential difference across a battery is always in one direction from positive to negative terminal. EMF is the source of current that drives electrons to travel in the circuit.
Potential difference across any conductor is product of current passing through it and the resistance of the conductor. It is directly proportional to the current. If the current changes direction, then potential difference will become negative or will be positive in the other direction. A potential difference is created due to the electrons traveling in a conductor.
Ohms' law is that the current passing through a resistor (conductor) is proportional to the voltage difference across the conductor. Or, in other words the voltage difference across a conductor is directly proportional to the current passing through it. This law applies at a constant temperature.
V α I
V = I R where R is the proportionality constant and is called the resistance
To measure resistance, connect a battery with known emf to the given resistance. Connect an ammeter in the circuit in series with the battery and the resistor. Connect a larger resistance in series with the given resistor. Connect a voltmeter in parallel with the conductor (or resistor, whatever we call it).
Now switch on battery and measure the voltage and current in the voltmeter ammeter respectively. Apply the equation to calculate the current through the resistor and the potential difference across resistor. Resistance is found by the law.