Both heavy water (D2O) and regular light water (H2O) are used in different types of reactors. The advantage of a heavy water reactor is that unenriched uranium fuel can be used. This is because the heavy water is a much more efficient moderator than light water. A greater number of stray neutrons can be slowed down enough to cause fission in U235. The D2O moderator makes up for the greater abundance of the neutron capturing U238. U238 is much more abundant than U235. U235 actually has to be made by seperating it from the ore using filters.
Btw D2O occurs naturally too, just in very small amounts. It is also seperated from regular water using some special process involving towers where the difference in weight is used to seperate it. The D stands for deuterium and is an isotope of hydrogen.
