Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids and gases from water. The goal is to produce water fit for a specific purpose. (Source: Wikipedia)
The input (how contaminated) and the output (how clean) will determine what technology to use and when.
Methods to clean can be filtering/sedimentation, chemical, flocculation, osmosis, UV or boiling. Methods are used at the source, along the chain or at the moment of consumption.
There are a huge number of providers for each and every technology. All technologies are aiming at solving the problem with varying potential to contribute to the global challenge.