What is the difference between an aquifer and an aquitard?

Question

What is the difference between an aquifer and an aquitard?

Answer 1

An aquifer is a permeable rock layer from which you can extract water, and they usually have an aquitard along them which is not permeable.

Answer 2

An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well.

An 'aquitard' is a geological formation of layers comprised either of clay, with tiny connected pores, or on non-porus rock that resrict water flow from one aquafer to another.