When we hear the word radiation, it’s tempting to picture huge explosions and frightening mutations. But that’s not the full story — radiation also applies to rainbows and a doctor examining an X-ray. So what is it, really, and how much should we worry about its effects?

 Radiation is the emission, propagation and transference of energy through any medium, in the form of electromagnetic waves or particles.

There are different types of radiations: the Electromagnetic radiations and Nuclear radiations. 

The Electromagnetic radiations lack enough energy to break the links that hold together the atoms of the medium they irradiate. The Nuclear radiations have enough energy to produce ionizations of the atoms in the irradiated medium or matter. They range from X-rays to cosmic radiation.

Source: - Youtube