Underground Water = Ground Water
Ground water is simply water under the ground where the soil is completely filled or saturated with water. This water is also called an "aquifer." Ground water moves underground from areas where the elevation is high, like a hilltop, to places that are lowland areas. Water movement is slow and might move anywhere from less than a millimeter up to a mile in a day.
Where the water table meets the land surface, a spring might bubble up or seep from the ground and flow into a lake, stream woodland, or the ocean. Ground water that meets the land surface also helps keep rivers, streams, lakes and wetlands filled with water.