A jail is a place where people are confined while they're being punished for a crime. If you end up getting a mug shot and put behind bars, chances are you are In jail.
To arrest someone and put them in a jail cell is to jail them. Jail prisoners are often simply waiting for their trials — they've been charged with a crime, but they haven't yet been found to be guilty (or innocent). Jail was originally spelled gayhol, and in Britain it's also spelled gaol. This can be traced back to the Medieval Latin gabiola, from Late Latin caveola and the Latin cavea, "cage or enclosure."