Radiation is the emission or transmission of energy as waves or particles through space or through a material medium which is able to penetrate various materials and is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Radiation processing can be defined as exposure of materials with high energy radiation to change their physical, chemical, or biological characteristics, to increase their usefulness, and safety purpose, or to reduce their harmful impact on the environment. Ionizing radiation is produced by radioactive decay, nuclear fission, and fusion, by extremely hot objects, and by particle accelerators. The radiation coming from the sun is due to the nuclear fusion; therefore, we are living in a natural radioactive world. Radioactive substances are common sources of ionized radiation that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, and photons, respectively. Alpha rays are the weakest form of radiation and can be stopped by paper. Beta rays are able to pass through paper but not through aluminum. Gamma rays are the strongest radiation. They are able to pass through paper and aluminum, but not through a thick block of lead or concrete. Alpha and beta radiation are the high energy subatomic particles where gamma radiation is a form of high energy electromagnetic waves. This review presents the fundamental introduction of radiation, the three types of radiation, and their applications.