The water that fills cracks, voids, and other openings in beds of rocks, sand, and soil is called groundwater. It is found in soils and sands that are able to retain the water — much like a sponge holds water.
It used to be that groundwater was considered limitless but now we know it is not. As we withdraw water from beneath the earth at rates higher than it is replenished we not only reduce the storage capacity of the aquifer but we activate faults and cause surface level subsidence, or sinking.
Surface water is water on the surface of the earth like rivers and lakes. In Montgomery County our surface water comes from Lake Conroe. Use of surface water allows us to avoid irreversible and costly sinking of our homes that comes from over production from our aquifers.
An aquifer is a subsurface geological formation. It's a body of porous sediment or clays that are saturated with groundwater. We bring groundwater to the surface through wells.
Hydrogeologists use graphics like this one to depict the aquifers in the Gulf Coast aquifer system.
In Montgomery County most of our water comes from the Evangeline (the green aquifer in this picture), the Chicot (the yellow one) or the Jasper (the orange one).