"I say to-may-to and you say to-mah-to" does not apply to jails and prisons. Yes, the words "jail" and "prison" are often used interchangeably by the press and sometimes even in legal blogs, but here's the truth: They are completely different animals. Just ask someone about to be sentenced for a crime.
TL;DR (Too Long; Didn't Read)
Jail and prison are two completely different types of incarceration, run by different agencies and housing different types of inmates.
She'll Spend Her Life in Jail
If you ever listen to the news, you will have been indoctrinated into the belief that jail is prison and prison is jail. Politicians and media use phrases like "she'll spend the rest of her life in jail," or "they'll send him to jail and throw away the key." But those things are very unlikely to happen.
Jails are local incarceration facilities, usually run by a city or county sheriff's office. They are used as short-term holding facilities for people likely to be there only a few days or weeks, like people who were just arrested and are awaiting a court appearance, or those waiting for trial or sentencing. Jails also house people convicted of misdemeanors who are serving sentences less than a year in jail. They are brought there after sentencing. Obviously, unless someone is expected to die quite rapidly, it is unlikely that person will spend her life in jail.
Jails are busy places, and new detainees arrive every day. Some just pause there for 24 hours until they are released after a court appearance. Others make bail, go to pretrial services, are placed in probation or are released on their own recognizance by promising to appear in court. Some are actively drunk or injured from brawling or mentally ill, brought to the jail because the police do not know where else to take them. That means that the jail is busy, and intake is challenging. But the stay is always short-term.
Prisons and Those Imprisoned There
In contrast, prisons are institutional facilities. Run by either the state or the federal government, they are built to house longer-term inmates, those convicted of felonies and serving sentences longer than a year. Most criminals are incarcerated in prisons. Those convicted of breaking federal laws are sent to federal prisons, state offenders are sent to state prisons.
Prisons are built to house many inmates for the long term, some for life. The inmates are serving time for more serious crimes, including assault and murder, and are considered more dangerous than those brought to jail. Security measures are much greater than for jails, and the rules enforced rigorously.
On the other hand, jails don't have many amenities for inmates since they aren't there for very long. But prisons have many more. They may offer libraries, substance-abuse help, vocational training, recreation, work release programs, an infirmary and halfway house services.