Thorium is a silvery metallic element with weak radioactive properties. While it will sustain a fission chain-reaction directly, in the presence of neutron bombardment thorium reacts until a fissile uranium isotope (U-233) is produced. So while not itself fissile, it is considered "fertile".
Most nuclear reactors rely on enriched uranium (U-235) or plutonium (PU-239) to generate power, but there are a few that operate with Thorium, in what is called the Thorium-Uranium fuel cycle. This process has some interesting benefits over the traditional U-Pu cycle, though disadvantages exist as well.
Thorium cycles allow for thermal breeder reactors which release more neutrons than fast breeders, meaning that much more power can be generated with less material. Thorium cycles also do not produce the elements that are the highest cause for health concerns, such as americium, curium and plutonium, and the waste if far less toxic.
Model of India's thorium fueled Advanced Heavy Water Reactor, currently on the works.
Another huge advantage is its abundance, which is more than three times greater than Uranium in the planet's crust, though this is offset by uranium's ubiquity in sea water. Both India and China have much more significant reserves of minerals which contain thorium than uranium.