You must be familiar with the constituents of atoms: Electrons, protons and neutrons. Normal hydrogen has one proton and one electron but there are other kinds of hydrogen, including something called deuterium (symbol D) which is an isotope of hydrogen. The deuterium atom has one proton, one neutron and one electron. So it is a heavier than normal hydrogen (H) which does not have a neutron. Water made with deuterium rather than with normal hydrogen (D₂O rather than H₂O) is called heavy water.

Deuterium occurs naturally, but in very small amounts, and in usually extracted by normal water using an electrochemical process (by passing a charge through water). It is used for processing nuclear materials and was initially used to make the materials for atom bombs, but has many peaceful uses as well, particularly in generating nuclear energy.

Heavy water is D₂O instead of H₂O, where D stands for deuterium, and an isotope of hydrogen.Instead of just having a proton and an electron, D has a proton, a neutron, and an electron and is thus about twice as heavy as a normal hydrogen atom.

As far as I know, the use of these to produce plutonium wasn't until after WW2. Plutonium was necessary for higher-powered atomic bombs (rather than the isotopes of uranium used in the bombs dropped on Japan). But by then, hydrogen bombs had already been developed, and plutonium bombs were probably mainly used to set off these even more devastating hydrogen bombs (which have tritium as nuclear fuel, which can also be produced by a process involving heavy water).

D₂O is naturally occurring in only very small quantities. More commonly seen is HDO, which is hydrogen, a deuterium, and oxygen. But the concentration isn't zero. There are several other uses for high concentrations of heavy water, such as neutrino detection (for studying fundamental particle physics), some applications in NMR (nuclear magneto resonance used for physics/chemistry as well as the fundamental process in conducting an MRI of your body parts), in nuclear power plants in production of fuel and as a neutron regulator to make controlled nuclear fission possible, and it is essentially non-toxic to humans with properties similar to water but easy to keep track of as it goes through the system, so sometimes it's used in metabolic research.

Like the uranium actually used to make the bomb, the element hydrogen also has more than one isotope. These are: ordinary hydrogen (one proton, no neutron), deuterium (one proton, one neutron), and tritium (one proton, two neutrons). Heavy water is water (one oxygen, two hydrogens), in which one or more of the hydrogen atoms is actually a deuterium. The extra neutron adds to the mass of the molecule, making it heavier, and less chemically active.

Because deuterium occurs naturally, heavy water molecules also exist naturally, and ordinarily natural water contains a mixture of normal and heavy water, more normal than heavy because normal hydrogen is vastly more common than deuterium. Artificially high concentrations of heavy water can be manufactured in a laboratory, however, and this is what is used when heavy water is used for some engineering process, such as making a bomb.

The main exception is that deuterium (and thus heavy water) does not absorb neutrons. This is important for some kinds of nuclear reactors: heavy water both assists and cools the nuclear reaction without stopping it. Heavy water is used for generating plutonium for the same reason.

There is a third kind of hydrogen called tritium, which has two neutrons. Tritium is used in some fusion experiments and thermonuclear bombs. It is radioactive, and takes hundreds of years to decay.