Radioisotopes are atoms which have an unstable nucleus, meaning they will undergo radioactive decay.
The term radioisotope comes from "radioactive isotope". An isotope is an atom which has the same number of protons, but a different number of neutrons. For example, cobalt-59, with 27 protons and 32 neutrons, and cobalt-60, with 27 protons and 33 neutrons. Isotopes will chemically interact identically, but they have different physical properties.
Radioisotopes emit different forms of radiation when they decay. These are alpha (#alpha#), beta (#beta#), and gamma (#gamma#).
A stable isotope is a nuclei which does not experience radioactive decay. Natural radioisotopes are radioisotopes which are found in nature, the most well-known being uranium. Artificial radioisotopes are radioisotopes which are artificially manufactured.
Although dangerous if handled without caution, radioisotopes have many applications. The most useful application is in medicine, where they are used to diagnose and treat various disorders, such as tumors.