Q: Why is a jailer one who jails, while a prisoner is one who’s imprisoned?
A: “Prisoner” once meant the opposite of what it means today. Back in the early 1300s, according to the Oxford English Dictionary, it meant “one who keeps a prison.”
But beginning in the late 1300s, it was also used to mean the guy on the wrong side of the bars. This is the meaning that has survived to our own time.
The suffix “er,” when added to a noun (like “jail” or “prison”), forms a new noun that can mean “one who is in charge of” or merely “one connected with” the original noun.
This is how we got derivative nouns like “jailer,” “prisoner,” “officer,” “mariner,” “carpenter,” “villager,” and many, many others.
The “er” can also be added to a verb to create a word for a person who performs the action represented by the verb (as in “write”/”writer” and “run”/”runner”). The new word is known as an “agent noun.”
Although “prison” and “jail” are often used interchangeably, in the US the term “prison” generally refers to a place for confining people convicted of serious crimes while “jail” refers to a place for confining people awaiting trial or convicted of minor crimes.