Electromagnetic energy is generated at the subatomic level; when atoms and their particles vibrate-something that occurs in any matter above absolute zero-the electrons in its “electron cloud” are oscillating (they accelerate and decelerate periodically), engendering electromagnetic energy.
The discharge of electromagnetic energy is known as electromagnetic radiation. From a collection of particles radiation is “emitted” in all directions at a variety of wavelengths and intensities.
The radiation of electromagnetic energy should not be confused with other types of radiation, specifically ionizing radiation-or radioactivity-though radioactivity can produce high frequency electromagnetic energy as well.
This video (4:13) explores electromagnetic radiation.