All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The scholars classified water with regard to its purity into three categories:
1- Water which is free from any restrictions: which is the water whose colour, taste and smell did not change with any outside effect. The ruling on this water is that it is pure in itself and it is a purifier, like rain water, snow water, hail, dew, sea water, springs, and Zam-zam water. And the water which is changed by what exists in it, or because of its long stay, or remaining still, or by what is emanated from it.
2- Water whose colour, taste or smell was changed with an outside effect, like dough, oil, soap, camphor, and perfumes. This kind of water is pure in itself, but there is a difference of opinion among the scholars with regard to it being a purifier or not.
3- Impure water, which is water with which impurity was mixed, and thus changed one of its characteristics. The ruling on this kind of water is that it is impure and that it is not permissible to use in acts of worship, for purification, to perform ablution, or Ghusl (ritual bath). It is also not permissible to use it for drinking or cooking.
However, some scholars divided water into two categories only: pure and impure. There are many definitions and differences about this issue and whoever wants more details on this, he should refer to the chapters on purity in the books of Fiqh (Islamic jurisprudence).
Allaah Knows best.