According to a definition used by the U.S. Bureau of Justice Statistics in its report, Prison and Jail Inmates at Midyear, jails are locally-operated correctional facilities that confine persons before or after adjudication. Inmates sentenced to jail usually have a sentence of 1 year or less. (This can vary by state.) Jails also incarcerate persons in a variety of other categories, such as persons being held pending arraignment, trial, conviction, or sentencing; those who have been returned to custody following violation of the terms of their release on probation or parole; and persons being transferred to the custody of other criminal justice/correctional authorities.

Prisons are operated by either a state or the federal government, and they confine only those individuals who have been sentenced to 1 year or more of incarceration. Generally, persons sentenced to prison have been convicted of a felony offense.

http://www.cdc.gov/nchstp/od/cccwg/difference.htm