Electromotive force (emf) is the force that causes current to flow from a higher to a lower potential. The term "electromotive force" is used for historical reasons, even though emf is not a force at all.
Any circuit with a constant current must contain an emf-producing source. Examples of emf sources include batteries, electric generators, solar cells, thermocouples, and fuel cells. All these sources transform energy of some kind (mechanical, chemical, thermal, and so on) into electric potential energy and send it to the circuit to which the source is attached.
When a battery is connected to a load, charges travel from the negative terminal to the positive terminal of the battery. Negative charges (electrons) must be transferred from the positive terminal to the negative terminal in order for the emf source to maintain the potential difference between the two terminals. In order to maintain the potential difference, the emf source functions as a charge pump, pumping negative charges from the positive terminal to the negative terminal. This raises the charges' potential energy and, as a result, their electric potential. The force on the negative charge from the electric field is in the opposite direction of the electric field.
In order for the negative charges to be moved to the negative terminal, work must be done on the negative charges. This requires energy, which comes from chemical reactions in the battery. The potential is kept high on the positive terminal and low on the negative terminal to maintain the potential difference between the two terminals.
When there is no current flowing, the emf is equal to the work done on the charge per unit charge. Its unit is in volt since the joule serves as the unit for work and the coulomb serves as the unit for the charge.
The terminal voltage is defined as the voltage across the terminals of the battery when a current flows through it. It is always less than the emf due to the potential drop across the battery's internal resistance, which is defined as the battery's resistance to current flow. The internal resistance generally increases as a battery is depleted due to the oxidation of the plates or the reduction of the acidity of the electrolyte.
