An aquifer is an underground layer of rock that is capable of storing water. Holes, or "pores", in the rock allow water to reside within it. The size and quantity of holes in rock is called a rock's "porosity". Holes may touch together, allowing water to flow through the rock, a characteristic we call "permeability".
Click here for an experiment, that creates a simulated rock aquifer with porosity and permeability and demonstrates how water is stored and moves through rock.