Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials. It can cause damage to matter, particularly living tissue. At high levels it is therefore dangerous, so it is necessary to control our exposure. Radiation on the lower end of the EM spectrum, things like radio waves, heat, and visible light, are sometimes lumped together in a group called non-ionizing radiation. The nastier sounding types like ultraviolet rays, X-rays, and gamma rays on the high end of the EM spectrum are collectively called ionizing radiation.