Jails are run by the local municipal or regional governments and generally speaking only house inmates for sentences of less than a year. San Diego appears to be an outlier with that 6+ year figure.
A prison is run by the state and is geared towards those with much longer sentences.
California’s horribly overcrowded prisons were found to be unconstitutional (cruel & unusual punishment), so they were under a court order to fix it. The “solution” was to transfer lots of prisoners to county jails.
In Massachusetts You can get as long as a 2 1/2 year sentence in county jail. You also can't get a state prison sentence from a district court. You have to be indicted and sentenced in Superior Court to do state prison time. In theory You could be sentenced to 3 consecutive 2 1/2 year county jail sentences in a District Court, that is rare, I believe. Contempt of Court is an outlier that is served in a county jail, but with a possible life sentence.
Generally jail is where you are held before a trial, or for short terms for minor crimes, prison is for serving your sentence after your conviction. This varies by locality.
Jail is for misdemeanor sentences, less than 1 year.
Prison is for felony crimes, 1 or more year.
Jail is also used as a holding area till someone is transferred to prison.
In the US the difference is that jails house people before trial and for misdemeanor sentences (generally less than one year) and prisons are for felony sentences (generally greater than one year).
