EM radiation is created when an electrically charge particle, such as an electron, is accelerated by a force causing it to move. The movement produces oscillating electric and magnetic fields which travel, as an harmonic EM wave, at right angles to each other. EM waves travel at 299,792,458 meters per second in a vacuum (the highest possible speed into the Universe, also known as the speed of light).
The electromagnetic field propagating through the space as EM waves is also referred as electromagnetic radiation.
An EM wave is characterized by a frequency (or by a wavelength) and by an amplitude (or by an energy).
The wavelength is the distance between two consecutive peaks of a wave. This distance is given in meters (m) or fractions thereof. Frequency is the number of waves that form in a given length of time. It is usually measured as the number of wave cycles per second, or Hertz (Hz). It is wave speed=frequency*wavelength so that, an EM wave traveling at the speed of light, can be equally identified by its wavelength or by its frequency. The amplitude (i.e. the maximum oscillation of the EM field) provide the intensity (i.e. the energy) of the EM wave.
The classical theory describes the EM radiation as electromagnetic waves which represent the oscillations of electric and magnetic fields. In the quantum mechanics theory EM radiation consists of photons, quanta of the electromagnetic energy, responsible for all electromagnetic interactions.
As far as Earth remote sensing is concerned EM radiation represents the most important vehicle of information.