Freshwater refers to naturally occurring water on the surface such as bogs, ponds, lakes, rivers and streams, and underground in aquifers and underground rivers. Freshwater is characterized by having low concentrations of dissolved salts. The term specifically excludes seawater and brackish water.

Scientifically, freshwater habitats are divided into lentic systems, which are the stillwaters including ponds, lake,swamps and mires; lotic systems, which are running water and groundwater which flows in rocks and aquifers. There is, in addition, a zone which bridges between groundwater and lotic systems, which is the hyporheic zone, which underlies many larger rivers and can contain substantially more water than is seen in the open channel. It may also be in direct contact with the underlying groundwater.

The source of almost all freshwater is precipitation from atmosphere in the form of mist, rain and snow. A very small proportion is emitted from active volcanoes. Freshwater falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled.

In industrialized areas rain is typically acid because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, lorries, trains and aircraft and from the atmospheric emissions of industry. In extreme cases this causes acid rain which can has caused severe pollution of lakes and rivers in parts of Scandinavia, Scotland, Wales and the United States.

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