The purpose of a well is to drill into the earth to gain access to ground water.
Ground water is simply water that flows beneath the earth's surface.
Many people mistakingly perceive this to be in large lakes or flowing rivers buried deep underground. Ground water is actually water that is completely saturated into soil, rock, or other geological formations. This water is in constant motion as it slowly seeps under ground. This can be compared to water in a sponge. If water is pored onto a sponge it will slowly saturate the entire sponge.