An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include: an aquitard, which is an impermeable layer along an aquifer, and an aquiclude (or aquifuge), which is a solid, impermeable area underlying or overlying an aquifer. The surface of saturated material in an aquifer is known as the water table.
For more, visit the Wikipedia entry for aquifer, URL: http://en.wikipedia.org/wiki/Aquifer