What is the spectrum of light?

The light spectrum refers to the quantitative differences in light - for example, what is the actual difference between red light and blue light, and what causes them to have those colors? While the term "light" is commonly used to refer to visible light only, the light spectrum can extend to include all types of electromagnetic radiation, and so it is a way of talking about the different properties of photons themselves.

Photons are massless particles that carry electromagnetic energy in a manner similar to both particles and waves. Photons can represent a broad range of energies; low-energy ones are found in sources such as radio, and high-energy ones are found in sources like X-rays, with a variety of different "states" in between, including visible light. The fact that different energies and properties of photons will change their appearance and interactions is the source of their identity along a spectrum. Technically, a spectrum refers to a range of continuous values, any of which might be assumed by a member of that spectrum. For light, any energetic value of the light spectrum may be assumed by a photon, but its location on the spectrum will determine its properties, and the photon will retain that identity for its lifetime.  

To refer specifically to the visible light portion of the spectrum, conversationally this refers to the colors and associated photon energies ranging from red to blue that humans can normally see. In terms of wavelengths and energies, this is about 400 to 700 nanometers, or about the range of one to two electrons worth of charge.