Ionizing radiation is radiation that changes the structure of individual atoms by ionizing them. The ions produced in turn ionize more atoms. Substances that produce ionizing radiation are called radioactive.
Radioactivity is a natural phenomenon. Nuclear reaction take place continuously on the sun and all stars. The emitted radiation travels through space, and a small fraction reaches Earth. Natural sources of ionizing radiation also exist in people and in the ground. The most common of these are uranium and its decay products.
Non-ionizing radiation ranges from extremely low frequency radiation through the audible, microwave, and visible portions of the spectrum into the ultraviolet range.