Linux systems have 7 Run Levels and they are as follows.
0 – system halt
1 – single user mode (system maintenance mode)
2 – multiuser mode without network access
3 – full multiuser mode with network access
4 – not used
5 – multiuser mode with X windows
6 – reboot or restart
Each run level in linux has a very specific purpose.
Run level 1 is also known as system maintenance mode and at this level the system prompt appears without root level password. It should be used only when it is absolutely necessary for system maintenance.
Run level 4 is not used and it can be used by the user to customize his own level.
Note: Solaris Run Level is different from Linux run levels.