There are all kinds of emf or voltage sources. What they do is to provide some kind of non-electrical force Femf. This force will act on some mobile charge carriers in the voltage source, and causes excess positive and negative charges to accumulate at the positive and negative terminals respectively. An electric field, directed from the positive terminal towards the negative terminal, is thus established inside the emf source.
Now that this electric field is established, any positive charge that is “lifted” from the negative to the positive terminal is going to be gaining EPE. Basically, the non-electrical force Femf must do work against the electrical force FE to ferry charges across the electric field. The EPE gained per unit charge is called the emf of the voltage source.
The emf of a voltage source is measured in volts. 1 V corresponds to 1 J C-1. For example, in a 3 V dry cell, 3 J of chemical energy is converted into 3 J of electrical (potential) energy for every C of charge passing through the battery.
 E.g. in a chemical cell, it is the “chemical force” pushing the ions in the electrolyte. In a generator, it is the magnetic force pushing the electrons in the armature windings.
 Don’t be misled by its name. An emf may be associated with an electric field and forces, but it is fundamentally a voltage, not a force.