A photon is an elementary particle. Photons have zero rest mass which allows long distance interactions. The photon is massless, has no electric charge and is stable. A photon has two possible polarization states. Similarly to other elementary particles, photons are best explained by quantum mechanics and wave-particle duality.
A photon moves at the speed of light and its energy and momentum are related by the equation E = pc, where p is the magnitude of the momentum vector p. The energy and momentum of a photon depends only on its frequency or inversely, its wavelength:
E= ℏw=hv= hc/λ
Where k is the wave vector, w = 2πv is the angular frequency, and ħ = h/2π is the reduced Planck constant.
Photons have applications in modern day technology. The classic photomultiplier tube exploits the photoelectric effect and photons landing on a metal plate. This principal is used in fire detectors. Other detectors use the ability of photons to ionize gas molecules causing a detectable change in conductivity.