An aquifer is a body of rocks or sediments capable of supplying groundwater at a useful rate. Conversely, a zone that that will hold water but not transmit it fast enough to be pumped from a well is called an aquitard or aquiclude. Gravel, sand, fractured rocks or karstified limestone massifs are good aquifers if groundwater is present, due to their high porosity.
The field of hydrogeology (or geohydrology) studies aquifers and their properties.
An aquifer is called an unconfined aquifer if there is no confining layer restricting the upper surface of the zone of saturation at the water table. If a confining layer is present, the aquifer is called a confined aquifer. For example, if a layer of gravels is embedded between two layers of non-porous clay, it potentially constitutes a confined aquifer. When the water of a confined aquifer is under pressure, it is said to be in artesian conditions. Water in artesian systems tend to rise to about the height of the water table in the recharge zone (the zone where the precipitation infiltrates to the surface to move down to the groundwater system),. When a well is drilled, the water of artesian aquifers reach the surface without pumping, creating an artesian well.
When water is pumped from a well, a cone of depression forms in the water or artesian pressure surface. A large cone of depression can alter the direction in which groundwater moves within an area. Overpumping of an aquifer causes the water level to lower continuously with time, which necessitates lowering the pump settings or drilling deeper wells. These adjustments are often costly, and they may or may not work, depending on the hydrologic conditions. The quality of groundwater may be also be degraded if it is extracted from deeper water containing more dissolved minerals.
Aquifer basics - USGS