A well is shaft that has been dug to access underground water, generally for the purpose of consumption. This water comes from a layer called the aquifer, which is made of water bearing sand, rock, or other material that filters the water.
An aquifer can be defined as a "saturated, permeable geologic unit that is capable of yielding economically significant quantities of water to wells and/or springs" ("The Compendium of Hydrogeology", Porges, Robert E., Hammer, Matthew J., 2001). Aquifers can be diverse geologic units, such as unconsolidated mixtures of sands and gravels, fractured bedrock, and weathered limestone. Water enters these aquifers via a process termed "recharge", which can come from rainfall, flow from streams and rivers, and in some cases leakage from overlying aquifers.
With respect to self-sufficiency, most "do it yourself" wells tap into shallow, unconsolidated sand and gravel aquifers. This is a benefit to self-sufficiency, but consideration should be give to the proximity of the well to septic tanks and leach fields, surface water bodies (e.g. rivers and ponds), and other potential sources of pollution.
Drilling a Well
A well is "dug, bored, or drilled excavation, or driven shaft, typically cylindrical, with the purpose of observing or withdrawing water from an aquifer or for injection of fluids below the ground" ("The Compendium of Hydrogeology", Porges, Robert E., Hammer, Matthew J., 2001.).
Most residential wells are constructed using low carbon steel (i.e. mild steel) or polyvinyl chloride (PVC). An "open bottom" well is un-perforated pipe, driven to a certain depth, allowing water to flow up into the pipe, known as well casing. Sometimes the well casing is perforated after it is driven or drilled into place, to allow more water into the well. The perforations are usually placed opposite aquifers that will yield water to the well, ideally. As wells become more sophisticated, the well casing is mated with pre-perforated well casing or well screen. This is engineered to allow more water into the well, while lending a degree of control to reduce the presence of sand in the pumped water.
Well diameters vary from 4 inches upwards, and will control the diameter (and therefore the flow rate) of the pump that can fit in the well. Most residential wells, yielding between 5 and 100 gallons per minute, will range from 4 to 10 inches in diameter.
Do it yourself wells often use PVC and steel well points, which are short sections of small diameter metal well screen, to allow water in and reduce the amount of sand entering the well.