Atoms can form ions by either gaining or losing electrons. When an atom gains one or more electrons it has a negative charge and is called an anion. When an atom loses one or more electrons it has a postive charge and is called a cation. Ionization is when an atom is struck with enough energy to knock an electron off of it, causing it to have a +1 charge. If an atom absorbs a photon with enough energy then ionization will occur.
Radiation is any form of energy or particles that starts in a center and moves outwards. Examples include all forms of light (also called electro-magnetic radiation) and alpha particles and beta particles. Some forms of radiation are non-ionizing: they cannot cause any atoms or molecules to lose an electron. These include radio waves, microwaves, infrared light and visible light. All of these forms of radiation are made of photons traveling at the speed of light. You should not look directly at the Sun because the light from the Sun is so bright it can burn your eyes and blind you. This is not because it is ionizing but because it is so intense.
Another category for radiation is called ionizing radiation. This is radiation that is energetic enough to knock electrons loose from atoms it hits. Some forms of electromagetic radiation are ionizing: ultraviolet, x-rays, and gamma rays. These forms of light have such short wavelengths, and such large energies, that they can ionize atoms. Another form of radiation is made of massive particles (photons do not have any mass). There are two kinds: alpha-particles and beta-particles. Alpha-particles are high-speed helium-4 nuclei with a mass of 4 amu and a charge of +2. Once an alpha-particle loses its very high speed and gains two electrons it becomes an ordinary atom of helium, which has two protons and two neutrons in its nucleus. Beta-particles are high-speed electrons with a mass of 5.49 × 10–4 amu and a charge of –1. All forms of ionizing radiation are capable of damaging DNA and causing mutations. Ultraviolet light from the Sun causes sun burns because it ionizes atoms in the skin, causing damage. Only gamma-rays, alpha-particles, and beta-particles are considered nuclear radiation because they are the only forms that come out of atomic nuclei. Nuclear radiation is dangerous but so are x-rays and ultraviolet light because they, too, can cause ionization.