In the United States custody system jails are detention facilities run by a city or a county, whereas prisons are run by the federal government or by each state. In practice, this means that every state and county runs its jails and prisons differently. Moreover, the laws governing these facilities vary widely.
A jail will hold people that are waiting for trial. Meaning that anyone arrested and awaiting to be released on bail bond or to see the judge will be held in a jail. Such a person can also be held in local substations. These municipal holding facilities are meant to save the officers' time. They don't have to drive arrestees out of town to county jails. However, someone arrested for a felony will be taken directly to a county jail.
Likewise, people sentenced to less than one year, usually for misdemeanors, will serve their sentence in jail. Prison is the place where people are confined after sentencing, generally for felonies.
Usually, jails and substations are not vast. Yet up to a few thousand inmates at a time can be held in some of the larger county facilities (LA for example). On the other hand, prisons hold hundreds, even thousands of inmates.
Detention Regime in Jails
Jails are usually crowded, reaching or exceeding their design capacity. Accordingly, the officers tend to be less understanding. There are not enough amenities, inmates are lucky if they have TV or some small library. Time is spent by talking with other inmates, playing cards, or other games. Also, there are no rehabilitation or education programs.
Those held in jail compose an eclectic mix, from people that have parking fines to murder suspects. In larger jails, inmates are separated according to gang affiliations. Therefore, a jail might be a more dangerous place where people need to be careful and watch their back.
Jail inmates have access to medical services for illness or injuries. If the jail is larger, it will have a medical section. Also, the person in need can be taken to a hospital. As a rule, the county will sustain medical costs.
A jail's kitchen is overseen by a cook, but meals are prepared by low-risk inmates.
Detention Regime in Prisons
Prisons range from minimum security to high-security ones. A minimum security prison has no fences and the officers don't carry firearms. It might even look like an apartment building. The most secure prison is the supermax one. Currently, there is only supermax prison in the United States, created in Florence, Colorado. In there, inmates are housed in a single cell and don't have access to many areas. These prisons are housing high-security inmates. As they require more personnel, they are expensive to operate.
In usual prisons, inmates tend to associate with the same type of offenders, for example, robbers would stick together. Although the prison holds people convicted of more serious crimes, officers tend to be more understanding or professional. Prisons are violent places, but they tend to be more organized, not as random as a jail.
Those held in a prison can earn substantial privileges, for example, limited access to a computer or better working conditions. Nearly all prisons have rehabilitation or educational programs and facilities where inmates can work. As for their final outcomes, unfortunately, statistics prove that most of the former prisoners don't reform.
On the whole, a jail may resemble a motel, these are agitated places, defendants, visitors, and lawyers are always coming and going. Whereas a prison might be seen as a secluded hotel better organized, with more educational and social options, where people rarely have contact with the outside world.