It has a larger than normal amount of the hydrogen isotope called deuterium (2
H or D, also known as heavy hydrogen). Ordinary water has the common hydrogen-1 isotope (1
H or H, also called protium). This makes up most of the hydrogen in normal water. The presence of deuterium gives the chemical different nuclear properties, and the increase of mass gives it different physical and chemical properties compared to normal "light water". Heavy water is used as a neutron moderator in some nuclear reactors, such as CANDU reactors. It is a more efficient neutron moderator than regular water, allowing the use of unenriched uranium as fuel. Pure heavy water is not radioactive, because deuterium is a stable isotope, but heavy water that has been through a nuclear reactor is slightly radioactive.
The chemistry of heavy water is different enough that people, and most living things, cannot survive with heavy water in place of ordinary water. Small amounts are not toxic to people. It is common for people to drink several grams of heavy water for metabolic experiments without causing illness.