A prison is a place in which people are confined and deprived of a range of liberties. Prisons conventionally are institutions authorised by governments and forming part of a country's criminal justice system, or as facilities for holding prisoners of war. A prison system is the organizational arrangement of the provision and operation of prisons.
There are a variety of other names for prisons, such as a prison-house, penitentiary or jail (in British English, sometimes spelled gaol although pronounced in the same fashion). There are, too, many colloquial terms for prisons - such as clink, hoosegow, lockup, lockdown and slammer—and imprisonment—doing time, bird, porridge.
However, in the United States at least, jail is generally used for facilities where inmates are locked up for a relatively short time (either while awaiting trial or serving a sentence of one year or less upon conviction for a misdemeanor), while prison and penitentiary typically denote a place where inmates go to serve long terms after having been found guilty of a felony. In Massachusetts, some jails are known as houses of correction. In Washington some adult prisons are called reformatories, while in other states this is reserved as a term for a prison of the juvenile justice system.
Prisons in the criminal justice system
In the domain of criminal justice, prisons are used to incarcerate convicted criminals, but also to house those charged with or likely to be charged with offences. Custodial sentences are sanctions authorised by law for a range of offences. A court may order the incarceration of an individual found guilty of such offences. Individuals may also be committed to prison by a court before a trial, verdict or sentence, generally because the court determines that there is a risk to society or a risk of absconding prior to a trial. The nature of prisons and of prison systems varies from country to country. Common though by no means universal attributes are segregation by sex, and by category of risk.
The availability of incarceration as a sanction is designed to mitigate against the likelihood of individuals committing offences: thus prisons are in part about the punishment of individuals who transgress statutory boundaries. Prisons also can serve to protect by removing from society individuals likely to pose a risk to others. Prisons also can have a rehabilitative role in seeking to change the nature of individuals so as to reduce the probability that they will reoffend upon release.
Crime and punishment is a wide, very controversial and deeply politicised area, and so too are discussions of prisons, prison systems, the concepts and practices of imprisonment; and the sanction of custody set against other non-custodial sanctions and against the capital sanction, a death sentence. Some of these issues are discussed in the by country descriptions, below.
Prisons form part of military systems, and are used variously to house prisoners of war, enemy combatants, and those whose freedom is deemed a risk by military authorities. The Geneva Convention provides an international protocol defining minimum requirements and safeguards for prisoners of war. Again, particularly after the US led invasions of Afghanistan and Iraq, military prisons and prison systems are highly controversial.
Certain countries maintain or have in the past had a system of political prisons; arguably the gulags associated with Stalinism are best known. The definition of what is and is not a political crime and a political prison is, of course, highly controversial, and critics can be found to rebut the suggestion that any of the following are political prisons:
World prison populations
Over nine million people are imprisoned worldwide. The prison population in most countries increased significantly beginning in the 1990's.
By country, the United States prison population is the world's largest in absolute terms, at more than 2 million; 70% of imprisonments are drug related. It is second largest in relative numbers with 701 people per 100,000 incarcerated; only in Rwanda, where as of 2002, over 100,000 people were held on suspicion of participation in the 1994 genocide, is the relative figure larger.
Both Russia and China also had prison populations of 1 million or more in 2002. No data is available for North Korea.  (http://www.homeoffice.gov.uk/rds/pdfs2/r188.pdf),  (http://www.prisonstudies.org/)
The UK had 73,000 inmates in its facilities in 2003, with France and Germany having a similar number.
Prisons in the United Kingdom
Prisons in the United States of America
Leavitt, Fred (2003) The REAL Drug Abusers. Rowman & Littlefield.