It is a process in water is purified or cleaned by different processes.
Some methods of water purification are as follows-
-Chlorination of water
-Sand Filter
-pH adjustment.
Process of Water purification...
here are a number of methods commonly used to purify water. Their effectiveness is linked to the type of contaminant being treated and the type of application the water will be used for.
Filtration: This process can take the form of any of the following:Coarse filtration: Also called particle filtration, it can utilize anything from a 1 mm sand filter, to a 1 micron cartridge filter.Micro filtration: Uses 1 to 0.1 micron devices to filter out bacteria. A typical implementation of this technique can be found in the brewing process.Ultra filtration: Removes pyrogens, endotoxins, DNA and RNA fragments.Reverse osmosis: Often referred to as RO, reverse osmosis is the most refined degree of liquid filtration. Instead of a filter, it uses a porous material acting as a unidirectional sieve that can separate molecular-sized particles.
Distillation: Oldest method of purification. Inexpensive but cannot be used for an on-demand process. Water must be distilled and then stored for later use, making it again prone to contamination if not stored properly.
There are many means of water purification and each has it's own advantage and the area of the use age.
Filteration: When muddy or un consumable water is made to pass some layer say a cloth or filter paper we get clean water in his process and the left over particles are called residue. 
Decantation: Water above the sand is pored out and clean water is collected and this process is called decanation.
Sedimentation: Here the heavier mud particles settle down and clear water is poured out and this simple process is called sedimentation.
Alum is also added to dirty water which helps the dust and other paricles to settle down.Alumn hastens Sedimentation.