According to the quality of domestic water supplies Analysis Guide, published by the Department of Water Affairs and Forestry  the term "water quality" is used to describe the microbiological, physical and chemical properties of water that determine its fitness for a specific use.
Water by its very nature possesses unique characteristics, chief of which is its excellent dissolving capability. As water moves through its cycle of rainfall, runoff, infiltration, impounding, use and evaporation, it comes into contact with a vast range of substances which may be dissolved by the water.  The type and amount of these dissolved determine the properties or quality of the water.
In addition to the substances that are dissolved in water, some substances may not dissolve, but remain as small suspended or colloidal particles. These particles also affect the quality of the water. Some suspended substances may include micro-organisms. They are so small that they cannot be seen individually with the naked eye but they may have important effects on water quality.
Different water uses require different qualities of water. For example, water of a relatively poor quality may be fit for use as irrigation water but will not be fit for use as domestic water. On the other hand, water which is fit for domestic use may not be fit for industrial use such as boiler feed water where water of high purity is required.
Appearance and taste of water may be misleading. When water looks clean and tastes good many people will accept it as a good quality drinking water. This may be dangerous because the water may contain excessive amounts of harmful substances such as mercury or micro-organisms which may have both short-term and long-term health effects on consumers. These can only be quantified by a proper analysis of the water.
The need for water quality information may arise from a number of different sources, 
Such as:
        Environmental health officers who have to assess the quality of domestic water at the point of use or in the distribution system
        Water suppliers who have to assess the quality of surface or groundwater sources to be used as a supply for domestic water
        Treatment plant operators who have to monitor the performance of treatment processes and the quality of water produced.
Each one of these will have specific needs for the types of analyses to be performed, the frequency of analysis, the accuracy required.