A well is a hole in the ground, usually with reinforced walls of stone, brick, wood, pottery (fired clay) or concrete, that people use to get water. While many people today have running water in their homes, many others still get their water from wells.
If you dig a deep enough hole, pretty much anywhere in the world, the bottom will eventually fill up with water. This water originally comes from rain; when rain falls it seeps into the earth but eventually, when it sinks deep enough, it reaches a layer of either impermeable clay or impermeable rock; at this point the water can't flow down any further, so it stacks up.
The depth beneath the surface at which the earth contains so much water that, if you dig a hole, it will immediately fill up with water, is called the water table. If you have to dig down 10m to find water, people say that the water table is at a depth of 10m.
The level of the water table changes over time. Often, the water table is deeper during the dry season, and shallower during the rainy season, or after a rain storm (although these changes are not immediate - it takes time for the water beneath the soil to respond to changes above the surface). If a well is not deep enough, it could go dry just when it is needed most. The water table in a region can also drop if too much water is being withdrawn from wells, which can cause soil salinization, damaging the soil. Unfortunately, in many parts of the world the water table is sinking as a result of climate change.
Hand-dug wells are usually between 5m and 20m deep. They need to be at least 1.5m wide so a person can get in to dig.
The digging might be done with spades and pickaxes, or it might be done by ramming a pole down into the earth until the earth is broken up into small, loose pieces. The excess earth has to be carried out of the hole in buckets and dumped somewhere.
The problem with trying to dig a deep but narrow hole in the ground is that the hole quickly collapses in on itself. A common technique to solve this problem goes like this: first, a shallow hole is dug, perhaps 1 or 2 meters deep. Walls are built to support the edges of this small hole. These supporting walls extend a meter so up above ground level. They might be made of wood, brick and mortar, stone and mortar, or concrete. The shape of the structure is usually a circle, sometimes a square, and there is no 'floor'.
Once these supporting walls are complete, the hole is dug deeper, until it is possible to ram the structure further down into the earth. Then the walls are built up from the top until they once again reach a meter or so above ground level. The process is repeated again and again, until the well is deep enough.
Sometimes, sections of the well structure are pre-built, rather than being built up as the well is being dug.
Some wells have steps or a ladder, so people simply go down with a bucket and carry the water back up.
Other wells simply have a bucket and a rope; the bucket is lowered until it hits the water, tips over, and fills with water, and is the pulled back up. Pulling the water back up is awkward and difficult, and therefore people may use a windlass, pulley, bucket chain, shadoof or water pump instead.
While many hand-dug wells are very simple, some are very beautiful and monumental, such as the famous stepwells of India and Pakistan.