Heavy water in the narrower sense is water that consists of water molecules of the chemical formula 2H2O (or D2O): Both hydrogen atoms in the molecules are "heavy hydrogen" i.e. deuterium atoms. This molecule is called heavy water (molecule), deuterium oxide, deuterated water or water-d2. If there is one normal hydrogen and one deuterium atom in the molecule (formula HDO) then it is called semiheavy water (molecule) or water-d1.
Since deuterium is chemically identical to protium (normal hydrogen), heavy water generally behaves similarly to normal water; heavy water looks, feels, and tastes almost exactly the same as normal water. However, because a molecule of heavy water weighs more than a molecule of normal water, it freezes and boils at a slightly higher temperature than normal water. (Heavy water freezes at 3.8°C and boils at 101.4 °C, as opposed to 0 °C and 100 °C for normal water.)
Most living things cannot survive solely on a supply of heavy water. Many of the chemical reactions that occur in living things require absolute precision, in both amount of chemicals and the timing of their movement around various tissues. Heavy water, with its greater weight and, by extension, momentum, does not allow these reactions to take place. Thus, while heavy water is not poisonous, it cannot support life on its own.
By far the largest source of heavy water is from Earth's oceans. A small percentage of the water in the oceans is heavy water. Heavy water plants simply pump in seawater and distill the heavy water from the normal water.
Heavy water has essentially zero cross section for neutron capture. This means that it can be used effectively as a moderator in a Uranium fission chain reaction. (Sustaining a chain reaction requires that as few neutrons as possible escape or are absorbed by other materials. A controlled chain reaction requires that the neutrons from one fission occurrence bounce off of many moderator atoms before the next fission occurrence. Even a very small probability of absorption per collision will kill the reaction. The protons in ordinary "light" water have a significant probability of capturing thermal neutrons. When they do so, they turn into heavy hydrogen, that is, Deuterons. The hydrogen atoms in heavy water are already Deuterons, so they won't absorb further neutrons.) During World War II, the German Nazi government used heavy water to moderate their nuclear reactors in their experiments to try to develop an atomic bomb. However, a raid on a heavy water plant in Norway greatly hindered the German efforts. In contrast, the Americans working on the Manhattan Project learned that graphite could moderate nuclear reactions much more efficiently than heavy water. While the graphite needed to be refined to great purity, this was not as formidable as enrighing heavy water. Graphite had the added benefit of being a solid, making it far easier to handle. The Americans published misleading data on the neutron absorption cross section of graphite, in order to keep the Germans from realizing that it would make a good moderator.
However, being flammable makes graphite more dangerous to use. The most dramatic example of this danger is the disaster at the Chernobyl power plant. A mishap in a plant using heavy water can theoretically be a "self-solving problem," as it is non-flammable.