Deuterium is a stable isotope of Hydrogen with a natural abundance of one part in 7000 parts of hydrogen. The nucleus of deuterium has one proton and one neutron, whereas a normal hydrogen nucleus has just has one proton.
It is useful in nuclear fusion reactions, along with tritium, because of the larger rate of reaction (or cross section) and high energy yield of the D-T reaction. Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which was a source of some concerns during World War II, as Germany was known to have conducted experiments that could have created the first hydrogen bomb. This lead to an important Allied special forces operation to destroy a Deuterium production facility in Norway.
Deuterium is frequently used in biochemistry as a tracer to study reaction pathways in cell metabolism because chemically it behaves identically to ordinary hydrogen, however it can be distinguished from ordinary hydrogen by its mass. Biochemical reactions involving hydrogen can therefore be traced by using deuterium instead. Also, because of its greater mass, chemical reactions involving deuterium tend to occur at a slower rate than the corresponding reactions involving ordinary hydrogen.
Canada is the world's leading producer of Deuterium as it is a by product of the CANDU reactor.