Electromagnetic
radiation is a stream of photons, which are 'massless' particles each
traveling in a wave-like pattern and moving at the speed of light. Each
photon contains a certain amount of energy, and all electromagnetic
radiation consists of these photons. The only difference between the
various types of electromagnetic radiation is the amount of energy found
in the photons. Radio waves have photons with low energies, microwaves
have a little more energy than radio waves, infrared has still more,
then visible, ultraviolet, X-rays, and ... the most energetic of all ...
gamma-rays.
Actually, the electromagnetic spectrum can be
expressed in terms of energy, wavelength, or frequency. Each way of
thinking about the EM spectrum is related to the others in a precise
mathematical way. So why do we have three ways of describing things,
each with a different set of physical units? After all, frequency is
measured in cycles per second (which is called a Hertz), wavelength is
measured in meters, and energy is measured in electron volts.
Note:
Electromagnetic
radiation from space is unable to reach the surface of the Earth except
at a very few wavelengths, such as the visible spectrum, radio
frequencies, and some ultraviolet wavelengths.
Read:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
More about the Electromagnetic Spectrum:
http://imagine.gsfc.nasa.gov/docs/science/know_l2/emspectrum.html
Now read:
The Multiwavelength Universe
http://imagine.gsfc.nasa.gov/docs/science/know_l1/multiwavelength.html
Imagine the Universe Science:
http://imagine.gsfc.nasa.gov/docs/science/science.html