An aquifer is a saturated formation of earth material which not only stores water but yields it in sufficient quantity.
The water bearing strata or formation.
Thus an aquifer transmits water relatively easily due to its high permeability. Unconsolidated deposits of sand and gravel form good aquifers.
They provide appreciable quantity of water to move through itself under ordinary field conditions. These are the geological formations in which ground water occurs.
The availability of water from it depends upon the rates of withdraw and refilling. They play the role of both a transmission conduit and storage.
These are classified on the basis of their occurrence and field conditions.
An unconfined, also known as water table aquifer, is one in which a water table exists.
Recharge of this aquifer takes place through infiltration of precipitation from the ground surface. A well driven into it will indicate a static water level corresponding to the water table at that location.
A confined, also known as artesian aquifer, is confined between two impervious beds such as aquicludes or aquifuges.
Recharge of this, takes place in that area where it is exposed to the ground surface. The water in the confined aquifer will be under pressure. Hence peizometric level will be much higher than the land surface. If peizometric level attains a higher level than the ground surface, then a well driven into it will flow freely without any pump.
A confined aquifer is called a leaky if either or both of its confining beds are aquitards.
Reference : Engineering hydrology by K. Subramanya