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Electromotive force (EMF) is the electric "force" that drives an electric current, produced by batteries, generators, etc., which is quantified as volts. Where there is a voltage difference, there is an electromotive force equal to that difference.
EMF is not a physical force (as in F=ma), but is analogous to a pressure, i.e., it is a quality that can produce a force, the amount of force depending on the EMF (and other factors). EMF differs from measured voltage in that it refers to a quality of a source of electricity when no current is flowing but voltage measurement is generally carried out by measuring the electric current through a resistor of known resistance. EMF is the limit of such a measured voltages given higher and higher resistance and a measured voltage over a sufficiently resistance will be close to the actual EMF.