Electromagnetic radiation (EMR) is a form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space.
Effect on organisms
The effects of EMR upon biological systems depends both upon the radiation's power and frequency.
For lower frequencies of EMR up to those of visible light (i.e., radio, microwave, infrared), the damage done to cells and also to many ordinary materials under such conditions is determined mainly by heating effects, and thus by the radiation power.
By contrast, for higher frequency radiations at ultraviolet frequencies and above (i.e., X-rays and gamma rays) the damage to chemical materials and living cells by EMR is far larger than that done by simple heating, due to the ability of single photons in such high frequency EMR to damage individual molecules chemically.
Electric and Magnetic
EMR is a particular form of the more general electromagnetic field (EM field) that is defined as the field produced by moving charges.
EMR has both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation.
Energy and Momentum
EMR carries energy - sometimes called radiant energy - through space continuously away from the source.
EMR also carries both momentum and angular momentum.
These properties may all be imparted to matter with which it interacts.
EMR is produced from other types of energy when created, and it is converted to other types of energy when it is destroyed.
The photon is the quantum of the electromagnetic interaction, and is the basic "unit" or constituent of all forms of EMR.
The quantum nature of light becomes more apparent at high frequencies (or high photon energy). Such photons behave more like particles than lower-frequency photons do.
In classical physics, EMR is considered to be produced when charged particles are accelerated by forces acting on them.
Electrons are responsible for emission of most EMR because they have low mass, and therefore are easily accelerated by a variety of mechanisms.
Rapidly-moving electrons are most sharply accelerated when they encounter a region of force, so they are responsible for producing much of the highest frequency electromagnetic radiation observed in nature.
Quantum processes can also produce EMR, such as when atomic nuclei undergo gamma decay, and processes such as neutral pion decay.
(based on wikipedia article on electromagnetic radiation)