water is perfectly safe to drink. All it is is "pure water". DI
stand stands for "deionized" meaning that the water was purified by removing the ions and metals from the water. Other methods of purifying water include distillation, which is boiling the water and collecting the pure water in a distillation chamber, hence the name of the process. Also, RO
water or reverse osmosis water is produced by pushing water across a membrane using pressure. Water is pushed from a higher concentration of ions to a lower concentration of ions in this process, which is the REVERSE of the natural process called OSMOSIS.
Water having a "rough taste" means that you can taste the heavy metals such as iron. You'd know what I'm talking about if you have ever taken a drink from a drinking fountain with lime deposits by the spout.
Heavy water contains two deutirium atoms rather than the more commonly occuring hydrogen atoms. Deutirium atoms, are in fact heavier or more massive than hydrogen atoms because they have an extra neutron, as jrp mentioned. Also water is made up of a very small percentage of these molecules. Heavy water was also used in Germany for the development of nuclear bombs in the 1940's.