Dear WW,
Groundwater is the term used for water that is always present below the ground often adjacent to lakes, rivers, the sea shore or else trapped in aquifers. As you surmise the water table does go up and down depending on weather conditions, or if someone, as wwh says, draws off water in your area that causes a local dip in the water table around the pump out, or dewatering, point. I'm far from an expert as my only concern, professionally, is what to do if it contains minerals, such as sulphur, that attack concrete and damage building foundations, tunnels etc. However here is a link that contains some good stuff simply presented.
http://www.groundwater.org/GWBasics/gwbasics.htmIf you Google ground water (as two words) you'll find you can access The Ground Water Atlas of the United States. Have fun!
dxb
