4. Porosity and permeability
The porosity of the soil is the percent of the soil that is air space.  Porosity ultimately affects the amount of water a particular rock type can hold and depends on a couple of different factors;
Particle shape - round pebbles have greater porosity than angular ones sorting - when particles are all the same size porosity is greatest
The ability of the ground water to pass through the pore spaces in the rock is described as the rock's permeability.  Permeable layers of rock that store and transport water are called aquifers.  While porosity and permeability usually go hand-in-hand, though some porous rocks are not permeable and some impermeable rocks are porous.  Permeability is affected mostly by the size and arrangement of the grains in the soil.
Grain size - larger grains are more permeable
The Water Table
As rain and runoff enter the soil the water begins to fill the pore spaces in the ground.  The water will continue to work its way down until it accumulates  above an impermeable layer (bedrock).  Just like the soil horizons the areas of saturated an partially saturated soils can be defined as layers (bottom up) .