Runlevel | Runlevel Description |
| Runlevel 0 | The halt runlevel - this is the runlevel at which the system shuts down. For obvious reasons it is unlikely you would want this as your default runlevel. |
| Runlevel 1 | Single runlevel. This causes the system to start up in a single user mode under which only the root user can log in. In this mode the system does not start any networking or X windowing, X or multi-user services. This run level is ideal for system admins to perform system maintenance or repair activities. |
| Runlevel 2 | Boots the system into a multi-user mode with text based console login capability. This runlevel does not, however, start the network. |
| Runlevel 3 | Similar to runlevel 2 except that networking services are started. This is the most common runlevel for server based systems that do not require any kind of graphical desktop environment. |
| Runlevel 4 | Undefined runlevel. This runlevel can be configured to provide a custom boot state. |
| Runlevel 5 | Boots the system into a networked, multi-user state with X Window System capability. By default the graphical desktop environment will start at the end of the boot process. This is the most common run level for desktops or workstation use. |
| Runlevel 6 | Reboots the system. Another runlevel that you are unlikely to want as your default. |
source:http://linuxpoison.blogspot.com/2008/04/13578175807301.html