Pure and purifying water - absolute water. This water is pure in itself and purifying when it is used in purification bathing, Wudu, and ridding of najas filth
Pure water
pure but not purifying - used water. Like the water that has been used for Wudu. This water is still pure in itself but not purifying. So if this water is collected it cannot be used for purification or removal of najas filth, unless it's a lot of water. It can also be used for washing clothes that do not have najas filth on them. Like juice, coffee, tea, and the like.
Najas filthy water
small amount of water that came into contact with unexempted najas filth, even if the water's properties(taste, color, smell) do not change