Volts and Potential

The emf or electromotive force is due to the diference in energy levels of an electron at the two electrodes. If the change in energy for one coulomb of charge is one joule, the cell potential, denoted E, is one volt.
See the physics tutorial for more details.
Standard concentration is 1.0 M, standard pressure is 1.0 atm, and standard temperature is 25°C. A cell's potential is readily calculated if we know the cell conditions. If we measure the potential with all of the compounds at standard concentrations and pressures, the measured potential will be known as the standard potential, denoted E°. This value indicates the tendency of reactants to be converted to products at these standard states.

Each redox reaction is composed of two half reactions. If we could assign a potential to each half reaction, then we could readily add the potentials of each half reaction to obtain the cell potential. We can't measure an isolated half reaction's potential directly however, because the potential is the potential energy difference of the electrons in each compound. Instead, we compare the standard electric potential against a standard half cell reaction:
2H₃O⁺(aq) + 2e^- ==> H₂(g) + 2H₂O(l)
We assign this potential of 0.00 volts to this half reaction. It is known as the standard hydrogen electrode or S.H.E.