The criminal justice system has its own unique vocabulary. This link provides a glossary of terms from the U.S. Department of Justice that may be helpful to you in your understanding of the criminal justice system.
A jail is a local facility that is typically run by a city or a county. Jails house people who have been arrested by the police and are going to face criminal charges. Jails also house some persons who are serving a sentence for having been convicted of a crime. Usually, those persons serving sentences in jails are convicted of misdemeanor and other low-level crimes and serve usually short periods of time measured typically days and weeks.
A prison is a correctional institute run typically by State governments that houses only persons who have been convicted of crimes. Usually, prisons house only those convicted of more serious felony crimes who are serving prison sentences typically measured in years.
Jails contain a mix of both persons who have been convicted of a crime and persons who have been arrested by the police and either have been charged with a crime or may be charged with a crime.
The mix of jail populations varies by local jurisdiction in terms of the percentage of those who are serving a sentence versus those who are accused of a crime. No percentage necessarily indicia of a “problem.” A high percentage of those accused could mean that the locality has a robust and effective probation system that reduces the need for hard jail time of those accused.
Similarly, a higher percentage of those serving sentences in jail does not then mean the system of arrest and release is more effective in such jurisdiction.