A photon is a tiny packet of energy produced within an electromagntic spectrum. It has no resting mass, quite simply, it is a release of energy produced when an electron in an atom jumps from one elctron level to another. When this "stimulated" electron returns to it's "normal" position, the energy released is called a photon. Photons may take the form of visible light, but remember, visible light is only a small part of the overall electromagnetic spectrum. Depending upon frequency and wavelength, the liklihood the photon will be of an invisible nature is greater.