A continuum comprising all the various sources of electromagnetic radiation. Essentially, radio waves, microwaves, heat, visible light, ultraviolet light, X-rays, gamma rays, cosmic rays, etc., are all versions of the same phenomenon: energy radiated through space. (This is why sound waves are not typically considered part of the electromagnetic spectrum, since sound requires a medium—such as air or water—through which to propagate, whereas electromagnetic waves have little difficulty in radiating through the vacuum of space.)
This radiated energy is in the form of waves (light, however, creates some difficulty, since in some cases it tends to behave like discrete particles—or quanta—rather than waves, but color theory and many other sciences dealing with light require that it be considered a wave). All the various forms of this radiated energy differ in terms of wavelength and frequency, both of which are mathematically related to each other.
The variations in wavelength are what produce the different perceived effects of various types of radiation. The longest (and believed by some to be the most benign) wavelengths belong to radio waves, while the shortest and most damaging wavelengths belong to the gamma rays and cosmic rays. Electromagnetic waves, regardless of their wavelength or frequency, all generally have the same source: accelerating electric charges, which can either consist of tiny yet powerful electron jumps between energy states within atoms (which generate visible light, ultraviolet light, X-rays, and gamma rays), or by "extra-atomic" electron conduction, as in an electric circuit (which produce radio waves).
The wavelengths covered by the electromagnetic spectrum range from 0.00003 ?ngstroms (?) (or 0.000000000000003 meter) for cosmic rays, to 300 meters for radio waves.
Some of the common subdivisions of the electromagnetic spectrum and their respective wavelengths are:
See also Visible Spectrum