Short answers, pulled from the story.
A photon is an elementary particle that is a quantum of the electromagnetic field, including light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that move only at the speed of light in a vacuum, and they belong to the class of boson particles.
In 1905 Albert Einstein proposed that energy quantization is a property of electromagnetic radiation itself, modeling light as discrete energy quanta. He built on Max Planck's hypothesis, and Einstein received the 1921 Nobel Prize in physics for his explanation of the photoelectric effect.
The name photon derives from the Greek word for light, phos. Its use for the light quantum was popularized by Gilbert N. Lewis, who used the term in a letter to Nature on the 18th of December 1926.
Wave-particle duality means a photon shows both wave-like and particle-like behavior. When detected it registers as a single particulate unit, yet the probability of detecting it is calculated using wave equations, so a photon passing through a double slit lands at a point whose probability follows an interference pattern.
The photon is generally considered to have zero rest mass and is treated as strictly massless in current physical theories. The experimental upper limit on the photon mass is very small, on the order of 10 to the minus 53 grams.
Photons emitted from the Sun's photosphere take 8.3 minutes to reach Earth. Inside the Sun, photons scatter so many times in the radiative zone that radiant energy takes about a million years to reach the convection zone.