Answer to Question 1
Ground water is the subterranean water stored in open spaces between particles, or pores. Some is stored in fractures.
Answer to Question 2
Porosity and permeability tend to increase with depth into the Earth, as compaction reduces pore space and cements fill space. Crystalline rocks don't have pore spacethey are fractured, and fracturing is most common in the upper crust. Limestone and sandstone and unconsolidated sediments have high porosity; shale has low porosity.