Photons are massless particles travelling at the speed of light. Because of length contraction, a single photon is literally nothing more than an infinately thin wavefront. Because of time dilation, time for a photon literally stops. It's one of the reasons photons can go on forever.
Because they're massless, they are the embodiment of pure energy. As they travel through the graviton matrix, they're affected by the matrix in strange ways, including red shifting, refraction, and relativistic effects. It's because of the graviton matrix that they appear to exhibit wavelike phenomena, such as refraction and wave interference.
But they're not waves; nor are they particles. That's where the confusion has been for so long.
Rather, they're simply a small packets of pure energy,
frozen in time, each of which exists at it's own unique, but sharply defined, quantum level. Taken together, they can appear to have any particular frequency, but taken singly, they exist only at a precise quantum energy state.
This becomes important later when we get into how photons appear to be travelling at c to all observers at all velocities of c, regardless of the relative velocity of the photons' source.
More detail:
The photon is the guage boson of the boson family of elementary particles. It has no electric charge, can exist at two possible polarization states, and posseses a wave vector (which determines it's wavelength (function of it's energy state) and direction of travel). Interestingly, although it doesn't have mass, it does have the ability to impart momentum. This is due to the fact that it is energy and the graviton matrix, which I'll explain later.
Photons can be created by several processes. The simplest is when an electron of an excited atom drops to a lower orbital, releasing a photon. You see this as the deep red coming from the burner of your stove (or blue, if you're using gas).
When photons are absorbed by matter, they in turn excite the electron according to it's
perceived energy state, and this is where relativity comes in, as the velocity between the emitter and the receiver is what determines it's perceived level of energy.
That's enough on the photon. Again, this thread is primarily about gravitons, so I'm not going to reinvent (or restate) the wheel talking about Bose-Einstein statistics, Fock states, etc. (although bosonic algebra does play a role in the graviton matrix with respect to how forces such as weak, strong, electric, and magnetic are communicated over distances).
Wiki:
Photon