Water is composed of hydrogen, two parts and oxygen, one - H2O. As it comes from the tap it is often treated in some manner, often with chlorine, to make it fit for human consumption. The catchment clearly affect its 'hardness' and flavour.
Water is used extensively in cooking both as a medium to impart heat (boiling) or as an ingredient, often replacing stock or other flavoured items. It is also used to dissolve things such as salt or to hold, for example, cornflour in suspension.