Groundwater is held in sediments and rocks below Earth's surface. In a natural groundwater system, water that has infiltrated the ground seeps slowly through the pores and fractures in rocks and sediments. Some rock/sediment types store water much better than others (i.e. water can more easily be extracted). A unit of rock or sediment that allows water to move through the pore spaces is called an aquifer (see Figure below). Relatively impermeable layers, call aquicludes, help to keep groundwater contained in the aquifers.

Aquifers that are bounded, top and bottom, by aquicludes are called confined aquifers. Because water in confined aquifers is trapped between impermeable rock layers, it cannot freely move up or down but is forced to flow laterally. Unconfined aquifers, on the other hand, are bounded by an aquiclude only on the bottom are, thus, generally present near the surface of the earth.

Groundwater is replenished primarily by rainwater that infiltrates the ground. This is known as 'recharge'. The degree to which groundwater is a renewable resource depends on the balance between removal rates (generally due to pumping) and recharge rates. Because water can move freely downward into unconfined aquifers, they recharge fairly rapidly when it rains. However, many of these shallow aquifers have become too polluted to be useful as drinking water. Most deep aquifers are confined aquifers that can only be recharged at certain locations. For this reason, confined aquifers have very slow recharge rates and can hardly be considered renewable on human time scales.