Aquifer: An aquifer is a saturated formation of earth material which not only stores water but yields it in sufficient quantity relatively easily due to its high permeability. Deposits of sand and gravel form good aquifers.
Aquitard: It is a formation through which only seepage is possible and thus the yield is insignificant compared to an aquifer. A sandy clay unit is an example of aquitard.
Aquiclude: Formations like clay which is highly porous but not permeable due to very small size of pores.
Aquifuge: It is a geological formation which is neither porous nor permeable. Massive compact rock without any fractures is an aquifuge.