In popular parlance of many countries, the term jail (gaol) is considered synonymous with prison, although legally these are often distinct institutions: typically jails are intended to hold persons awaiting trials or serving sentences of less than one year, whereas prisons host prisoners serving longer sentences.

A criminal suspect who has been charged with or is likely to be charged with a criminal offense may be held on remand in prison if he or she is denied, refused or unable to meet conditions of bail, or is unable to post bail. This may also occur where the court determines that the suspect is at risk of absconding before the trial, or is otherwise a risk to society. A criminal defendant may also be held in prison while awaiting trial or a trial verdict. If found guilty, a defendant will be convicted and may receive a custodial sentence requiring imprisonment.

Prisons may also be used as a tool of political repression to detain political prisoners, prisoners of conscience, and "enemies of the state", particularly by authoritarian regimes. In times of war or conflict, prisoners of war may also be detained in prisons. A prison system is the organizational arrangement of the provision and operation of prisons, and depending on their nature, may invoke a corrections system. Although people have been imprisoned throughout history, they have also regularly been able to perform prison escapes.

History

For most of history, imprisoning has not been a punishment in itself, but rather a way to confine criminals until corporal or capital punishment was administered. There were prisons used for detention in Jerusalem in Old Testament times. Dungeons were used to hold prisoners; those who were not killed or left to die there often became galley slaves or faced penal transportations. In other cases debtors were often thrown into debtor's prisons, until they paid their jailers enough money in exchange for a limited degree of freedom.

Only in the 19th century, beginning in Britain, did prisons as we know them today become commonplace. The modern prison system was born in London, as a result of the views of Jeremey Bentham. The notion of prisoners being incarcerated as part of their punishment, and not simply as a holding state till trial or hanging, was at the time revolutionary.

The first "modern" prisons of the early 19th Century were sometimes known by the term "penitentiary" (a term still used by some prisons in the USA today): as the name suggests, the goal of these facilities was that of penance by the prisoners, through a regimen of strict disciplines, silent reflections, and maybe forced and deliberately pointless labor on treadwheels and the like. This "Auburn system" of prisoner management was often reinforced by elaborate prison architectures, such as the separate system and the panopticon. It was not until the late 19th Century that rehabilitation through education and skilled labor became the standard goal of prisons.