EMF, or electromotive force, is the energy per unit charge supplied by a source to drive electric current in a circuit. It's measured in volts and represents the potential difference created by converting other forms of energy into electrical energy.
The first method of EMF generation is electromagnetic induction. When a conductor moves through a magnetic field, or when the magnetic field around a conductor changes, an EMF is induced according to Faraday's law. This principle is used in generators and dynamos to convert mechanical energy into electrical energy.