Linux Standard Runlevels
This reference explains the standard Linux runlevels:
0 – Halt
Shuts down the system.
1 – Single-User Mode
Maintenance mode without networking.
2 – Multi-User Mode (No networking)
Multiple users can log in, but no network services are available.
3 – Multi-User Mode with Networking
Full multi-user mode with network capabilities (no GUI).
4 – Undefined / User-definable
Reserved for custom configurations by the user.
5 – X11 (GUI Mode)
Multi-user mode with graphical interface (X11).
6 – Reboot
Reboots the system.