Heavy Water is water that has a slightly different composition than regular water.

Normal water is made up of 2 Hydrogen and 1 Oxygen atoms. Heavy water is also made up of 2 Hydrogen and 1 Oxygen atoms, but the Hydrogen atoms are a Hydrogen isotope* called Deuterium instead of regular Hydrogen.

(*In simple terms, an isotope is a variation of an element with the same number of protons and a different number of neutrons. It’s the same element, just a little different.)

The result is water that is heavier, i.e., has more mass, than regular water. For instance, ice made from heavy water would sink in regular water. The freezing and boiling points of heavy water are also slightly different than regular water. It freezes at 3.81°C (38.86°F) and boils at 101.42°C (214.56°F).

Heavy Water is used in nuclear reactors because the additional mass slows down fast-moving neutrons released by nuclear fission so they have more time to react with the nuclear fuel.

Heavy Water is also slightly toxic, since the additional mass that is a benefit in nuclear reactors is bad for your body. You don’t want it to take longer for the chemical reactions in your body. They are designed for regular water.

However, it would take ingestion of a lot of heavy water to cause problems. Your body is mostly water, you see, and you’d have to replace a lot of the regular water in your body with heavy water before you saw trouble.

I guess that’s why you can buy Heavy Water Vodka.

Source: The Straight Dope and wiki