Radiation is the emission, propagation and transfer of energy in any medium in the form of electromagnetic waves or particles.
Living beings have coexisted with radiations since their origins. Without the radiation from the Sun, life would not have existed on Earth and without infrared radiation we would not be able to keep ourselves warm. As well as these natural sources of radiation, human beings have been capable of developing various devices based on the use of radiations. We use radiations when we listen to the radio, talk on a cell phone, heat our breakfast in a microwave, toast bread or get an X-ray.
Electromagnetic waves or radiations can be classified as ionising radiations, if they have sufficient energy to ionise atoms in the matter they pass through (e.g., X-rays), or as non-ionising radiations, if they do not have sufficient energy to break the links between the atoms and cause their ionisation, such as ultraviolet rays and radio, TV and cell phone waves.
Radiations in the form of particles with mass (e.g., alpha radiations) are included within the ionising radiation category since in all cases they can cause ionisation.