A jail is used to temporarily detain those who are suspected or convicted of a crime. It is used for the short-term, usually to hold those awaiting trial or to hold those convicted of low-level offenses that have sentences of one year or less. A prison is a facility that holds convicts who have committed crimes the legal system deems especially serious (e.g., first degree murder) for more long-term sentences.
edit Length of Detention
The most notable difference between jails and prisons is that prison inmates have been tried and convicted of crimes, while those in jail may be awaiting trial, where they may yet be found innocent. A prison is under the jurisdiction of either federal or state governments, while a jail holds people accused under federal, state, county, and/or city laws. A jail holds inmates from two days up to one year.
The following video explains the differences between a jail and prison:
edit In the United States
Jails are usually run by sheriffs and/or local governments and are designed to hold individuals awaiting dispostion of their case, waiting for transport to a state prison system following conviction, or serving time after a misdemeanor.
State prisons are operated by the state where the person was convicted of a felony. Federal prisons are operated by the Federal Bureau of Prisons (BOP) and are designed to hold individuals convicted of federal crimes, such as tax evasion, bank robbery, or kidnapping, among others.
Jails operate work release programs, boot camps, and other specialized services. They try to address educational needs, substance abuse needs, and vocational needs while managing inmate behavior.
State prison systems operate halfway houses, work release centers, and community restitution centers — all considered medium or minimum custody. Inmates assigned to such facilities are usually reaching the end of their sentences.