Any water suitable for drinking is potable water.
Tap water is potable, or drinking, water. When it is introduced into a boiler or other non-drinking resources, it becomes NON potable
There are many ways, but the two most common are desalinization tablets and distilling the water. Reverse osmosis works well, as does water softeners. Both uses are elaborate in construction, but...
Could be anything. Chemicals, sewage, micro organism's, something you shouldn't drink.
Potable is drinkable like from the hose or kitchen sink Nonpotable is sewage