Based on the way you wrote this I get the understanding that the US system differentiates between jail and prison? In that case what's the difference?
Prison is where people who are convicted of felonies (crimes worthy of >1 year in custodial sentence) are sent. Jails are where people awaiting trial, or convicted of misdemeanors (crimes worthy of <1 year in custodial sentence) are sent.
Prisons are run by a state or by the Federal government, and are for holding persons who have been convicted of a crime and sentanced to a longer term.