Porosity is the percentage of a rock's volume that could potentially be filled with air or water. Sandstones and conglomerates have relatively high porosity.
Permeability is the ability of water to flow through rock.
Sand, gravel, sandstone, conglomerate, and limestone with a lot of caves all have high permeability. Find an example of a rock that is porous but not permeable. Clay and pumice are both very porous but not very permeable. An artesian well is a well that taps a confined aquifer and has a high enough water pressure that water rises all the way to the ground surface. Artesian wells form where permeable rock trapped between impermeable layers slopes downward, so that the water is forced to be under pressure.