Different rocks have different porosities. Rocks may have very high porosities but not let water move through them. The permeability of rock is its ability to transmit water. Depends on porosity and interconnectedness. Molecular attraction or surface tension inhibits flow particularly in small pore spaces.
Aquifers are rocks with high permeability.
Aquitards are impermeable. Water must migrate through the pore spaces of rock. Underground water moves under a hydraulic gradient - a slope to the water table. Hydraulic head - the difference in height of the water table between the recharge and discharge points. Springs Occur where the water table intersects the earth's surface.