what is the part of the ground where all of the pore spaces are filled with water?

I think you should re-phrase your question so it's a little clearer

An Aquifer. Below the Water Table.

This question isn't very well typed.
Below the crust(most outer most part) is the hard darker rocky layer
Then after several kilometers below is the water table ,about some 15 to 20 kms app.
Then there rocky pores filled with water molecules called the aquifer
It is a permiable layers that allows water to perculate along,and filters the water as well.that is why the pure water is gained from there... but more solublized entities like ionic and hydrates salts may perculate too. Giving rise to serios water pollution.