Shaykh Yasir Qadhi discusses the different branches of Fiqh in this lecture.
Acts of our religion and the consequent Fiqh can be classified into the following categories:
Haram
Makrooh
Mubah
Mustahab
Wajib
The Shaykh starts off with the Fiqh of purification or Tahara.
Categories of water:
Tahoor fluid - natural water - it is pue in itself and can purify other things.
Taahir fluid - it is pure in itself but does not purify other things.
Najas fluid - it is impure in itself which implies that it cannot purify anything.
We get a detailed explanation of the three types of water from the Shaykh and deserves a fair listen.