What are Photons?

Under the photon theory of light, a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum, have a constant speed of light to all observers, at the vacuum speed of light (more commonly just called the speed of light) of c = 2.998 x 108 m/s.

Source: milemathis.com

According to the photon theory of light, photons . . .

• Move at a constant velocity, c = 2.9979 x 108 m/s (i.e. "the speed of light"), in free space
• Have zero mass and rest energy.
• Carry energy and momentum, which are also related to the frequency nu and wavelength lambda of the electromagnetic wave by E = h nu and p = h /lambda.
• Can be destroyed/created when radiation is absorbed/emitted.
• Can have particle-like interactions (i.e. collisions) with electrons and other particles, such as in the Compton effect.

The term photon was coined by Gilbert Lewis in 1926, though the concept of light in the form of discrete particles had been around for centuries and had been formalized in Newton's construction of the science of optics.

In the 1800s, however, the wave properties of light (by which I mean electromagnetic radiation in general) became glaringly obvious and scientists had essentially thrown the particle theory of light out the window. It wasn't until Albert Einstein explained thephotoelectric effect and realized that light energy had to be quantized that the particle theory returned.

Source: http://physics.about.com/od/lightoptics/f/photon.htm