What is the difference between EMF and Voltage?

EMF stands for the Electromotive Force and it can be defined in a few ways:

1. It's the potential across the terminal ends of a cell.

2. Or in other words, the amount of energy that is given to a charge in in the circuit as it leaves the cell. 

When we talk about EMF we usually talk about a circuit powered by a cell with some internal resistance. All cells have an internal resistance that prevent them from outputing a constant Voltage. Some of this energy is lost as heat to the surroundings when it passes through the internal resistance of the cell.

When we talk about voltage we usually talk about the potential difference across two points in a circuit.

EMF can be found using the equation E = I(R+r).

Voltage can be found using the equation V = IR.

If we compare both equations we see that EMF and V are very closely related, where EMF adds the internal resistance to the equation to make up for the energy loss in the cell due to it's internal resistance.