The different prison categories refer to the prison's security level. In the United States, state prisons are divided into roughly five security categories: minimum, medium, close security, maximum and supermax. Federal prisons, according to the Federal Bureau of Prisons, use the equivalent security categories: minimum, low, medium, high and administrative.
According to Vince J. DeMaille of Incarceration 101, minimum and medium security prisons generally house low-risk inmates in dorm-like facilities with group showers. Guard presence is relatively light. The perimeter for medium security prisons has a double fence. Minimum security prisons may have a single fence or no fence at all, depending more on the remoteness of their location. Close security houses inmates in double cells, but often has open communal yards within double or triple fences. Maximum security prisons, where violent offenders are housed, keep prisoners in cells designed to hold a single inmate, and inmates are kept almost exclusively to a single cellblock. Inmates moved out of the cellblock are escorted and restrained. Supermax, the heaviest security level, isolates prisoners from almost all social contact in single cells, keeping them under near-constant surveillance.
Not all states have all levels of prison, and in many cases the same prison complex includes sections with different security levels. Each state has its own numbered prison categorization system, with the lowest level commensurate with a minimum security facility. The federal government has a total of six numbered prison levels, and local jails typically operate on an unnamed security level equivalent to close security.