This process is one more confirmation of Einstein's special theory of relativity. It is the energy gained by one electron when it moves through a potential difference of one volt. Lets look at an example of how this energy unit works. The rest mass of an electron is 9.11 x 10 -31 kg. This gives us the energy equivalent of one electron.
Physicists go one stage further and convert the joules to electron volts.
So if you ask a high energy physicist what the mass of an electron is, you'll be told the answer in units of energy. Quite easily, as long as everyone understands the system. Not only are mass and energy measured in eV, so is momentum. It makes life so much easier than dividing by c 2 or c all the time. There is more information available on units in relativistic physics.