Purification is the removal of impure elements from something. After purification, water is safe to drink. Most cities have a system of water purification, so people get clean fresh drinking water without any parasites or goldfish in it.
Purification is when things are cleaned and made pure. Salt water must undergo purification before it's safe to drink. Some filters do a purification of the air to reduce allergens. In some religions, purification is the removal of sin, which is another type of cleansing. For example, sumo wrestlers throw salt in the ring for purification, as part of the tradition of the Shinto religion.