Perpendicular sinusoidal superimposed electric and magnetic fields, electromagnetic radiation accounts for all visible light, radio waves, microwaves, infrared light, ultra-violet light, x-rays, and gamma rays. It travels at a constant speed and, unlike sound waves or water waves, requires no medium through which to propagate.

Edl = -dφB/dt

or in other words, that a changing magnetic field induces an electric field.

We can also take Ampere's Law, as corrected by Maxwell, and remove terms pertaining to charged particles to obtain the following version applicable to fields in a vacuum:

Bdl = μ0ε0 * dφE/dt

which basically says that a changing electric field induces a magnetic field.

With that in mind, imagine a system of an electric field and magnetic field wherein a change in one field results in a change in the other which reiterates and perpetuates the system. That's electromagnetic radiation: sinusoidal electric fields perpendicular to sinusoidal magnetic fields, the changes of each perpetuating the changes of the other. Because Maxwell's Equations have been modified to deal only with fields in a vacuum, the radiation requires no medium through which to propagate.

Another interesting facet of the unique properties of electromagnetic radiation is that its speed in a vacuum is constant. Through complex derivations (which I have neither the time nor the command of HTML to demonstrate), one can prove that the speed of electromagnetic radiation is

c = 1/(μ0ε0)½

Note that that speed is independent of reference frame; that is, every reference frame will view electromagnetic radiation traveling at exactly the same speed, hence all of special relativity. The speed of light in a vacuum is really universal regardless of whether the radiation travels through a vacuum, but when electromagnetic radiation travels through denser substances, it is continually being re-absorbed and re-emitted fractions of a second later by individual atoms. That, not a change in speed of light, accounts for the slower measured speeds.

Also, electromagnetic radiation is not just light, or at least not in the traditional sense. The visible light spectrum has a wavelength (distance from one electric or magnetic peak to the next) of 400 nm for deep violet light through 700 nm for deep red, but that's hardly the complete list of the masks electromagnetic radiation wears in the name of variety.

Here's the breakdown:

• Radio waves: from basically arbitrarily large wavelengths (though 106 m is about the largest you'll get in practice, as larger wavelengths require higher minimum amounts of energy and become extremely difficult to detect) through 1m
• Microwaves: from 1 m through 0.01 m
• Infrared: from 0.01 m through 7 × 10-8 m
• Visible light: Evenly distributed between 7 × 10-8 m and 4 × 10-8 m in the following order:
• Ultra-violet: from 4 × 10-8 m through 10-9 m
• X-rays: from 10-9 m through 10-12 m
• Gamma rays: 10-12 m through arbitrarily small wavelengths