Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations.
A formation of rock or soil is called an aquifer when it can produce a usable quantity of water. Two soil layers — unsaturated and saturated zones — can be distinguished below ground level. The unsaturated zone lies nearest the surface, and the gaps between soil particles are filled with both air and water. Below this layer is the saturated zone, where the gaps are filled with water. The depth at which soil pore spaces become fully saturated with water is called the water table.
Aquifers can be either unconfined (i.e. when only the bottom is confined by a layer of impervious material, so that the aquifer is in contact with the atmosphere through the unsaturated zone) and confined (i.e. when the aquifer has impervious layers both at the bottom and at the surface). Both types of aquifer have a recharge area where water infiltrates the ground surface to reach the aquifer. Groundwater recharge is dependent on precipitation and soil moisture conditions. Aquifers usually recharge slowly and mostly during the wet winter season.
For further details: UK Groundwater Forum
Groundwater over-exploitation occurs when groundwater abstraction exceeds recharge and leads to a lowering of the groundwater table. The rapid expansion in groundwater abstraction over the past 30 to 40 years has been due to new agricultural and socioeconomic development in regions where surface water resources are insufficient, uncertain or too costly. Over-abstraction leads to a lowering of the water table, loss of wetlands dependent on groundwater and deteriorating water quality. This is a significant problem in many European countries, in particular, along the coast where the population density and the need for water are high. Over-abstraction in these areas often leads to intrusion of saltwater into aquifers making groundwater unsuitable for consumptive uses.