Cheat Sheet for Linux Run Levels

"Standard" Linux uses the following run levels:

• Run Level 0 is halt (shutdown).
• Run Level 1 is single-user mode.
• Run Level 2 is multi-user mode (without networking)
• Run Level 3 is multi-user mode (with networking). This is the normal "terminal" mode. (I.e., before the display manager is run).
• Run Level 4 is undefined.
• Run Level 5 is multi-usermode with a GUI display manager (X11).
• Run Level 6 is reboot.

In Debian and its derivatives run levels 2 thru 5 are the same: multi-user mode with networking, and with a display manager if available.

• Run Level 0 is halt (shutdown).
• Run Level 1 is single-user mode.
• Run Level 2-5 is multi-user mode with networking and a GUI display manager when available.
• Run Level 6 is reboot.

Debian also adds Run Level S, which is executed when the system first boots.

Also see Wikipedia's article on run levels.