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A photon is the particle, wave packet, or "wavicle" by which energy in the form of electromagnetic rays is transmitted; it is a type of boson.
Electromagnetic rays or electromagnetic waves that scientists study range from radio waves with very long wavelength through to very short wave gamma rays or X-rays. A small subset of electromagnetic waves are what humans can see and what we call visible light. Photons are characterised by their (notional) frequency which is directly related to their energy. Paradoxically more energetic photons have shorter wavelength while less energetic photons have longer wavelength. There is no theoretical upper limit or lower limit on the frequency of electromagnetic rays.
Electromagnetic waves are also called electromagnetic radiation.
 Energy of a photon
The amount of energy carried by a photon depends entirely on its frequency ν:
- E = hν
The constant h, known as Planck's constant, is an extremely small number — only 6.6×10-34 Joule-seconds. This means a single photon's energy is really tiny. A single photon of frequency 5.4×1014 Hz, the frequency of visible yellow light, has a paltry 3.6×10-19 Joules of energy. You'd have to have 2.8×1018 such photons hitting you every second just to receive 1 Watt of light.
 Speed of light
The speed of a photon, in a vacuum, is constant (generally denoted c) and is the limiting speed of information transfer in the Universe. The theory of relativity was largely inspired by contradictions implicit in the invariance of the photon's speed in thought experiments.
 Ultimate decay product
The photon is one of the ultimate decay products of all other subatomic particles being itself immune to decay (at least, it's believed to be).