Gamma rays are high-energy light, higher-energy even than x-rays. They are created by nuclear reactions such as fission, fusion, or radioactive decay. Like any kind of high-energy radiation (even ultraviolet), they can alter DNA, which can in turn lead to a variety of other problems (e.g. cancer). Being as high-energy as they are, gamma rays are more dangerous than any other frequency of light, although there are ways to stop them.

Alpha and beta particles are not light at all. Alpha particles are helium atoms (two protons, two neutrons), and beta particles are electrons.