Run Levels
When the computer first turns on, it enters different run levels when the computer runs a Unix system.
The run levels on a standard Linux System are:
0: Shutdown
1: Single User
2: Multi User no Networking
3: Multi User with Networking (typical for servers)
4: User Define
5: Multi User with Networking and GUI
6: Reboot
A computer starts at run level 0 when it is off. When the user turns it on it will go from 1-5 for a desktop computer, or 1-3 on a server. When the user tells the computer to reboot it goes into run level 6.
You can use the init command (as root) to change the run level, though this is
seldom necessary.
