Ground water is water that infiltrates into the ground and slowly seeps downhill below the surface. Obviously, this is only possible if the rock, soil, or sediment layers are permeable meaning they allow water to pass through them. Permeability depends on the porosity (% of void space) of the material and the size and degree of interconnection of these spaces. 

As water percolates into a permeable layer, it fills the pores in the rock, displacing the air originally contained therein. The zone within which all the voids are filled with water is called the zone of saturation. Above the zone of saturation some of the spaces still contain air. This is called the zone of aeration. The upper surface of the zone of saturation is called the water table, the shape of which generally follows the shape of the ground surface, although in a more subdued fashion. 

Layers of rock that are saturated are called aquifers, while layers that prevent water from percolating through are called aquacludes. Within aquifers, ground water flows from high areas to low areas, forming a flow network. In areas with unconfined flow, water seeps through openings in rocks rocks as through a sponge, seeking its own level in response to gravity. In confined areas, water is trapped below a less or even non-permeable layer, an aquaclude and prevented from rising to its normal level. If this impermeable layer is breached, water will rise above the level at which it is held by this confining layer. Such a situation is referred to as artesian.