Heavy water is dideuterium oxide, or D2O. It is the same as normal water, H2O, except with two deuterium atoms instead of a hydrogen atom.
Due to its usefulness in nuclear weapons programs, heavy water is subject to government control in several countries. (In Australia, the Nuclear Non-Proliferation (Safeguards) Act 1987).
India is the world's second largest producer of heavy water through its Heavy Water Board (http://www.heavywaterboard.org/)
Heavy water is toxic, since it inhibits cell division, but only in very large quantities: approximately 50% of the water in one's body must be replaced with heavy water for poisoning to occur. The symptoms of deuterium poisoning are similar to those of radiation poisoning or chemotherapy.
Heavy water would be an ideal poison for killing someone with, since it is extremely unusual for forensic tests for it to be performed, and it would appear that the person was merely suffering from some mysterious illness; however, heavy water is very expensive, and requires a government license to purchase, making this method of killing someone less attractive in practice.