Electromotive force is the difference in electrical potential between two points that compels electricity to flow to an area of lower potential in order to equalize charge. According to the HyperPhysics website, the force is measured in volts and frequently abbreviated as "emf."
Electromotive force can come from magnetic induction or it can be generated chemically by batteries. When a magnetic field interacts with an electrically-conductive material, the field's repulsion causes the electrons in the conductor to move away, thus producing electrical current. The electrons then gather at one end of the conductor, creating an area of higher electrical potential, while the opposite end is positively charged. Attaching two wires and a component load to this new power source creates a basic circuit.
Batteries make use of oxidation reactions to strip electrons away from their interior and send them to the negative terminal, where they flow through the circuit, and then back through the positive terminal and through the battery. The most common type of battery used today is the alkaline battery, which uses potassium hydroxide as the conducting material. It reacts with a zinc sleeve to send electrons to the negative terminal.
The electromotive force is measured relative to zero potential; so positive voltage can go either way on a circuit with negative voltage taking the opposite direction.