A criminologist, or expert on criminal behavior, who specializes in penology knows all about prisons, including the management and rehabilitation of prisoners.
The ultimate root of this word is the Greek word poine, or "penalty," and that makes sense when you learn that the word refers to the branch of criminology dealing with prisoners and their management. An older spelling of the word is poenology, in which you can perhaps discern a relative of penology: "subpoena," literally "under penalty."