A good aquifer is one that carries a large amount of water and recharges quickly. Some aquifers are even under pressure so that little if any work is required to bring water to the surface. An aquifer is any body of groundwater into which a well can be dug.
Aquifers are buried porous rocks or masses of unconsolidated material, such as gravel, sand or silt, that are saturated with water. When one digs into such a rock or mass of material, water seeps into the hole, forming a well. When the layer of water-bearing material has porous materials above or beneath it, it's not under much pressure, and energy must be used to obtain the water. In artesian wells, however, the layer of water-bearing material is sandwiched between non-permeable materials, such as granite or clay. In these cases, the water is under pressure.
Aquifers have limited supplies of water and rely on precipitation to be refilled. Different aquifers refill at different rates depending on the amount of precipitation and the nature of the overlying materials. Human overuse of aquifers can deplete them or, in the case of coastal aquifers, cause saltwater to seep in, rendering them unusable for drinking or agriculture.