Radiation is anything that radiates away from something. In chemistry and physics, it refers to light, radio waves, X-rays, and gamma rays, as well as to particles such as neutrons, protons, helium nuclei (alpha particles) and electrons (beta particles).
Most forms of radiation we encounter are harmless or even necessary for life. Without light and heat, there would be no life. We use radio waves for communication, and they pass through us without harm.
In medicine, radiation usually refers to ionizing radiation. This is radiation which has enough energy to strip electrons away from atoms, forming ions. Since electrons are what form the bonds between molecules in our bodies, ionizing radiation can change those bonds in harmful ways, creating burns or damaging DNA which can cause cancer.
Radioactivity refers to something that happens to atoms that have too many neutrons in them in proportion to the number of protons. These atoms are unstable, and decay into more stable atoms by emitting radiation.
We saw earlier how radioactive carbon-14 decayed into nitrogen by changing a neutron into a proton and an electron (which speeds away as a beta particle). More famously, uranium goes through a series of radioactive decays into various elements that eventually turn into lead.
Radioactive elements produce many types of radiation as they decay. They can emit electrons, alpha particles, neutrons, neutrinos, and gamma rays (a form of high energy X-rays). They can also produce larger atomic nuclei that fly away as the atom splits.