The definition between a jail and a prison is very much depends on what state you are in.
For example in Texas, you've sat in jail for 6 month and receive a 1 year sentence, you will not be sent to prison, you will remain in the jail for the next 6 months. You can remain in 'jail' for years sometimes before being sent to a prison facility, even after sentencing.
In the USA, sentences of less than 1 year are misdemeanors and can be served in a jail. Sentences of longer than 1 year are felonies and are served in prison. Jails are operated under authority of the county in which they are located, prisons under the authority of state or federal government. This is obviously a summary rather than an exhaustive definition.
