The USGS Water Science School
Water in the ground is an intricate part of the water cycle. After rain soaks into the ground it begins to move (according to gravity and pressure).
For people, one very valuable reservoir of water underground are called "aquifers". Simply, aquifers are layers and areas of rocks below ground where all the cracks, crevices, and spaces between rock particles are full of water. The water is able to move through aquifers and people drill wells into them and pull the water out to use for their own uses.
Maybe you've heard of "artesian well water"? An artesian aquifer is an aquifer under pressure...the rock is being pressed from all sides, so pressure builds up. When people drill a well into an artesian aquifer the pressure inside can push water to the land surface by itself (no well pump needed). As an example, if you fill a plastic baggie full of water and then press it between your hands and stick a pin in it, water will come shooting out. I guess that would be an artesian baggie.
Here is a picture of an artesian well really spouting off in southern Georgia, USA. People have drilled a well into the aquifer (this one is under pressure) and water is moving so easily in the aquifer and there is so much pressure underground that water is shooting out at the land surface. Not all artesian wells are this powerful, as many of them result in just a mild flow of water to the surface.