The Quran says, Allah is pure and He will not accept any Ibadah (worship) except such is of purity. Generally speaking, purification is of two kinds - water and sand. However, for the benefit of the question which is of water, it is divided in to two - you can purify by ablution with water or ghusl (washing the whole body) with water.
A muslim is discouraged from performing salat, doing circumambulation (Tawaf), touching the Holy Quran and reciting it and entering the mosque when impure. This shows the importance of purification in Islam. No aspect of worship shall be done when in a state of impurity and if done will not be accepted. Allah says in the Quran, "If you are in a state of Janabah (after sexual intercourse), purify yourself.
There are certain types of water for purification, e.g, rain water, ice water, rivers, sea water, wells and fountains. Q25:48 says; "And we send pure water from the sky."
It suffices to say that a pure water is any water free from colour, smell or taste. However water that changes in colour or taste due to the after effect of salt, mud such as lakes or streams are considered to be pure and can be used for purification.