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.
A prison is run by the state and is geared towards those with much longer sentences.