Massachusetts becomes fifth state to make prison calls free

I am not from USA, so what is the difference between prison and jail?

In general, the idea is a jail is where people who are accused of a crime stay while they are awaiting trial if the nature of their potential crime warrants it, a prison is where people who have been convicted of their crime serve their sentences after the trial. But it is confusing because jails are often used for short sentences (on the order of a few days or months).

The US has 50 states. And everything like this is can vary by federal, state, county, and muni's. So...

Jail is for naughty people. Prison is for bad people.

Jails are usually run by counties and muni's and hold people before trial and serving sentences for minor crimes. Prisons are run by the feds and states and hold prisoners convicted of serious crimes.

Technically, a jail is for people not yet convicted, and prison is for the convicted. In practice, a lot of people just use the terms interchangeably.

and are housed interchangeably in big cities where most of the population experiences these facilities

Jails are also often used for people who are convicted of lesser crimes, with sentences under a year.

This is not correct. Yes, jail is people waiting for trial and usually convicted people with less than a year sentence. Prison is usually for people with a year or longer sentence. My state has a prison for younger drug offenders and other places for violent convicted criminals.