Deuterium is an isotope of hydrogen, the first element. Deuterium has one proton and one neutron. Hydrogen does not have a neutron, only a proton. Another isotope of hydrogen, tritium, has two neutrons. The chemical symbol for Deuterium is 2H but D is also used often.
Two deuterium atoms combined with an oxygen atom is sometimes called "heavy water." This is because it is like water (H2O), but heavier because deuterium has one more neutron in its nucleus. Heavy water is sometimes used in nuclear reactors. It is also used as a solvent for NMR spectroscopy. This is because it will dissolve the sample, like normal water, but it will not be detected by the magnet in an 1H NMR.