Plutonium is a radioactive element, commonly created when low-enriched or natural uranium is irradiated in a nuclear reactor. Several isotopes of plutonium have been identified, but plutonium-239 garners the most interest due to its viability as both a nuclear fuel and as weapons material.
Plutonium-239 is made when uranium-238 captures a thermal spectrum neutron and undergoes two beta decays. Like thorium, uranium-238 in this context is considered fertile, whereas plutonium-239 is considered fissile. Plutonium-239 will fission 62% to 73% of the time on neutron capture.
Plutonium is attractive for advocates of the fast reactor, because plutonium is very likely to fission when it captures a fast neutron. However, the neutron cross section, or the probability of capturing a neutron, is much smaller in the fast spectrum. Plutonium-239 has the lowest critical mass of any fissile material, which makes it particularly well-suited to weaponization.
Plutonium-240 is the result when plutonium-239 captures a neutron without fissioning. The ratio of plutonium-239 to plutonium-240 determines the grade of plutonium; a high ratio is weapons-grade because plutonium-240 has a chance of spontaneously fissioning, which can destroy the plutonium in a nuclear weapon.
Plutonium-238 is made in a more complicated manner. A nucleus of uranium-235 captures 3 neutrons in succession, and undergoes 2 beta decays. Because of its comparatively short half-life (87 years) and alpha decay (ionized helium atom, or 2 protons and 2 neutrons), it is useless in weapons, but is ideal for devices called radioisotope thermoelectric generators. These devices use the heat and alpha decay of the radioisotope to generate small amounts of electricity over a very long time. This is the power source of NASA's long-term space probes like Voyager, Galileo, and Curiosity.