Services are generally processes that wait for a request to perform an operation or provide information and then do so. The System Settings → Server Settings menu provides menu items that let you administer certain services. The menu also provides the Services menu item associated with the Service Configuration tool, which lets you start and stop services and configure which services run at various runlevels. Red Hat Enterprise Linux and Fedora Core support seven runlevels:
Runlevel 0 is associated with a system shutdown.
Runlevel 1 is associated with the state known as single-user mode. It is generally used to troubleshoot or configure the system from a state that does not provide network services. It vaguely resembles Windows safe mode.
Runlevel 2 is associated with a state that provides access to the network but does not provide network services to remote hosts. Like runlevel 1, it’s primarily used for troubleshooting and configuration.
Runlevel 3 is a normal system operating mode. It features a text-based login prompt.
Runlevel 4 is reserved; that is, it is not used.
Runlevel 5 is a normal system operating mode. It features a graphical login screen.
Runlevel 6 is associated with a system reboot.
Runlevels 3 and 5 are normal system operating modes. Other modes are used transiently (levels 0, 6), for troubleshooting (levels 1, 2), or not used at all (level 4). When you associate a service with a runlevel, the service is ...