Aquifer

An aquifer is an underground  layer where the material contains water. That can be less solid material like sand, gravel, clay or silt, but it can be rock as well, as long as the rock allows water to get in (that means that it is water-bearing).
Groundwater can be taken out of the ground by digging a well. The study of water in aquifers is called hydrogeology.

 Groundwater in rock formations 
One kind of groundwater is found in underground rivers in caves where water flows freely underground. This may occur in eroded limestone areas known as karst topography, but those are only found in small parts of the Earth.

Usually, groundwater is found in the pore spaces, or tiny openings, of rocks under the ground.  The pores in the rocks are saturated, or filled up, with water, much like a kitchen sponge.  The groundwater  can be pumped out for agricultural, industrial, or municipal uses.

If a rock unit of low porosity is highly fractured, it can make a good aquifer (via fissure flow), if the water can get through it. Porosity is important, but, alone, it does not make an aquifer. The Deccan Traps (a flood basaltic lava) in west central India are good examples of rock with high porosity but low permeability, so they are poor aquifers. The (Upper Cretaceous) Chalk of south east England has a reasonably high porosity, but low grain-to-grain permeability. Much of its good water-yielding characteristics being due to micro-fracturing and fissuring.

 Examples 
The Great Artesian Basin in Australia is arguably the largest groundwater aquifer in the world. (over 1.7 million km²). It plays a large part in water supplies for Queensland and remote parts of South Australia.
The Guarani Aquifer with an area of 1.2 million km² is shared by Brazil, Argentina, Paraguay and Uruguay.
The Ogallala Aquifer of the central United States is one of the world's great aquifers. It underlies portions of eight states, contains primarily fossil water from the time of the last glaciation.  Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals.
The Nubian Sandstone Aquifer System (NSAS) is the world’s largest known fossil water aquifer system. 

 Exploitation 
Aquifers can occur at various depths. Those closer to the surface are not only more likely to be exploited for water supply and irrigation, but are also more likely to be topped up by the local rainfall. Many desert areas have limestone hills or mountains within them or close to them which can be exploited as groundwater resources. If more water is taken out than get in again one speaks of over-harvesting. Along the coastlines of certain countries, such as Libya and Israel, population growth has led to over-population which has caused the lowering of water table and the subsequent contamination of the groundwater with saltwater from the sea (saline intrusions). In these cases the aquifer contains brackish water.

When water seeps (comes slowly) out of the ground, it is called a seep or "spring (water)". It usually means there is an aquifer under the ground.

 References