Purified water refers to all types of water from which chemicals are removed via a variety of different processes. The simplest of these processes are available to consumers with water filters that help sort out some of the chemical compounds found in tap water. Other more expensive filters use a reverse osmosis technique, which helps shunt chemicals in the water to the side while leaving the remaining water free of most commonly found chemicals. Distilled water and deionized water are also considered purified.
Distillation of water generally means boiling the water so that any chemicals present are separated in the process. As water steam or vapor rise from a boiling batch, it is captured in tubes and allowed to cool back down to liquid state. Such a process can remove many chemicals from water, since they won’t turn into a vapor state. Often, water is double distilled.