Filtration is a process used to separate solids from fluids, in this case from water, by introducing a medium for the water to flow through but which the solids cannot flow through. The separation is not 100% complete, and the degree of completion depends on the pore size and thickness of the medium.

A simple example of a filter is a coffee maker, where the coffee grounds are filtered through a simple paper filter in the coffee maker.

A water filter is a device which removes impurities from water by means of a fine physical barrier, chemical process and/or biological process. Filters are used to cleanse water for irrigation, drinking water, aquariums, and swimming pools.

Water can be cleaned using a variety of different types of water filters, both for drinking and other purposes. Filtration forms a key stage in many waste water and drinking water treatment systems. In the case of drinking water, filtration tends to be carried out by the supplier prior to the water's entry into the distribution system. Additional filtration may be carried out by consumers at their premises using any of several types of domestic water filter.

Domestic water filters are primarily used to remove chemicals and metals such as chlorine and lead. Water filters can be used for preventative health reasons in the case of harmful substances, or for reasons of aesthetics, to improve the taste, color or odor of drinking water. Domestic water filters can be attached directly to a tap, attached to the domestic water supply before the tap, or filters can be used at the point of use or in a portable unit.