Roughly Americans use prison to refer to places where people are held after being sentenced, and jail to refer to where people are held before sentencing. Or sometimes jail refers to short sentences.
In England we'd tend to use prison or jail for prisons and "on remand" for people awaiting trial or sentencing, and "police cells" for other stuff.
In the US a lot of prisons have high populations of people with mental illness. See, for example, the use of prisoners to care for other prisoners who have dementia.
Jail is for pre-trial sentencing and sentences under one year. So many of the people have not been convicted. Jails spend most of their energies sorting people into various risk categories. There are few if any 'treatment' programs in jails.
Prisons are where people actually serve meaningful time. Everyone is in for at least a year and everyone is an actually convicted criminal, not just someone awaiting trial.
> Jail is for pre-trial sentencing and sentences under one year.
In California, specifically, since the recent prison realignment adopted to address Federal court orders with regard to California's overcrowded state prisons, that is substantially less true than it used to be; many non-serious, non-violent, non-sex offenders sentenced to more than one year are now sent to county jails, and population pressure resulting from that has reduced pre-trial detention in many county jails, as well.
There is some wiggle room. The one-year thing is more to do with how much time one is expected to serve, rather than the sentence. Time already served can also alter the math. So someone spending 6-months in jail pre-trial and then sentenced to 14 months would normally be sent back to jail as they only have 8 months remaining. And to your point, where it is very unlikely that someone will server their entire sentence prior to parol, they will normally do that time in a jail.
This isn't always a bad thing. Some convicts would rather not be moved to a new facility. Jails are normally closer to the place of arrest (usually near home) and by the point sentencing happens families have already developed some sort of routine for visitation. Sending them out of town, even if to a better facility, is not always in the inmate's best interest.
http://www.hg.org/article.asp?id=31513
In England we'd tend to use prison or jail for prisons and "on remand" for people awaiting trial or sentencing, and "police cells" for other stuff.
In the US a lot of prisons have high populations of people with mental illness. See, for example, the use of prisoners to care for other prisoners who have dementia.
http://www.nytimes.com/2012/02/26/health/dealing-with-dement...