In response to some comments, it might be fair to state that there has developed an entirely new branch of physics over the last 40 years. This is called Stochastic Electro-Dynamics or SED physics.
This branch of study is exploring the role played by the energy intrinsic in the vacuum, called the Zero Point Energy (ZPE). Research in the SED physics line is turning up new answers to old problems, and allows an easy visualization of some otherwise esoteric mathematics. Included in this are revised concepts of mass. These concepts are based on the action of the ZPE on charged point particles such as electrons and quarks (which make up protons and neutrons). This approach was initiated by de Broglie and Schroedinger, and the initial introduction to the ZPE was paved by the work of Planck back in 1911.
The ZPE exists as an electromagnetic waves that are intrinsic to the structure of the vacuum. When these waves impact on a charged, massless particle such as an electron, it causes the electron to dance about. New Scientist for 3 February 2001, page 24, puts it this way. "So de Broglie and Schroedinger proposed that an electron is actually a point-like charge which jitters about randomly within a certain volume.... Haisch and Rueda believe that ... the electron's jitter is ... just like the Brownian motion of a dust particle bombarded by molecules in the air. 'Random battering by the jittery vacuum smears out the electron', says Haisch. ... 'A massless particle may pick up energy from [the jittering caused by the ZPE], hence acquiring what we think of as rest mass'." In fact the article concludes by saying that "If this were the case, all three facets of mass would be different aspects of the battering of the quantum vacuum."
Further work on this suggests that mass at the atomic level arises from the the motion of the charged point particles within a certain volume, while at the macroscopic level, we measure mass by the total energy content of the system. Gravity is also readily explicable by the action of the ZPE on charged point particles, but enough has been said to assure you all that my original comments were not without scientific basis. Further reading on this matter can be found from the California Institute of Physics and Astrophysics (CIPA) on their website is at
www.calphysics.org/inertia.html
Thanks again for your comments. I have written this on behalf of my wife, 'tuppence' who is very adept at explaining some parts of physics in layman's language. She has a decent grasp of the subject and until you do, I suggest you pay a little more attention to what is being said here.