Springs are natural point sources of groundwater discharge. Although springs emerge onto the surface and may be treated as surface water after they discharge, they are part of a sub-surface system of aquifers that follows hydrogeological principles.
A spring is the result of an aquifer being filled to the point that the water overflows onto the land surface. They range in size from intermittent seeps, which flow only after much rain, to huge pools flowing hundreds of millions of gallons daily. Springs are not limited to the Earth's surface, though.