Runlevel is a mode of operation in operating systems with UNIX system implementation.
They are different from one OS to another but in general are 7 (from 0 to 6).

Typical runlevels are assigned to:
- single-user mode
- multi-user mode without network services started
- multi-user mode with network services started
- system shutdown
- system reboot
See below the most common and their differences.

Standard runlevels:

0 - Halt - Shuts down the system.
S - Single-User Mode - Does not configure network interfaces or start daemons.
6 - Reboot - Reboots the system.

Linux - Standard:

0 - Halt - Shuts down the system.
1 - Single-user Mode - Mode for administrative tasks.
2 - Multi-user Mode - Does not configure network interfaces and does not export networks services.
3 - Multi-user Mode with Networking - Starts the system normally.
4 - Not used/User-definable - For special purposes.
5 - Start the system normally with appropriate display manager. (with GUI) -Same as runlevel 3 + display manager.
6 - Reboot - Reboots the system.

Linux - Debian:

S - Only run on boot (replaces /etc/rc.boot)
0 - Halt
1 - Single user mode
2-5 - Full multi-user with console logins and display manager if installed
6 - Reboot

Linux - Ubuntu

0 - Halt
1 - Single-user mode
2 - Graphical multi-user with networking
3-5 - Unused but configured the same as runlevel 2
6 -Reboot

Linux - Redhat/Fedora:

0 - Halt
1 - Single user (safe mode)
2 - Full multi-user with network enabled but most network services disabled
3 - Full multi-user, console logins only
4 - Not used/User definable
5 - Full multi-user, with display manager as well as console logins
6 - Reboot

Linux - Suse:

0 - Halt
1,S - Single-user
2 - Full multi-user with no networking
3 - Full multi-user without display manager
4 - Not used/User definable
5 - Full multi-user with display manager
6 - Reboot

Linux - Slackware:

0 - Halt
1 - Single-user
2 - Full multi-user NO display manager
3 - Full multi-user NO display manager
4 - Full multi-user with display manager
5 - Not used/User definable
6 - Reboot

Linux - Gentoo:

0 - Halt
1 - Single-user
2 - Multi-user, no network
3 - Full multi-user with display manager
4 - Aliased for runlevel 3
5 - Aliased for runlevel 3
6 - Reboot

UNIX - System V

0 - Shut down system, power-off if hardware supports it (only available from the console)
1 - Single-user mode, all filesystems unmounted but root, all processes except console processes killed
2 - Multi-user mode
3 - Multi-user mode with RFS (and NFS in release 4) filesystems exported
4 - Multi-user, user-defined
5 - Halt the operating system, go to firmware
6 - Halt the system, reboot to default runlevel
s,S - Identical to 1 (Single-user mode, all filesystems unmounted but root, all processes except console processes killed) except current terminal acts as the system console

UNIX - Solaris:

0 - Operating system halted; (SPARC only) drop to OpenBoot prompt
S - Single-user with only root filesystem mounted (as read-only)
1 - Single-user mode with all local filesystems mounted (read-write)
2 - Multi-user with most daemons started.
3 - multi-user, identical to 2 (runlevel 3 runs both /sbin/rc2 and /sbin/rc3), with filesystems exported, plus some other network services started.
4 - Alternative multi-user, user-defined
5 - Shut down, power-off if hardware supports it
6 - Reboot

UNIX - HP-UX:

0 - System halted
S - Single-user, booted to system console only, with only root filesystem mounted (as read-only)
s - Single user, identical to S except the current terminal acts as the system console[/cross]
1 - Single-user with local filesystems mounted (read-write)
2 - Multi-user with most daemons started and Common Desktop Environment launched
3 - Multi-user, nearly identical to runlevel 2 with NFS exported
4 - Multi-user with VUE started instead of CDE
5,6 - user-defined

UNIX - AIX:

0 - Reserved
1 - Reserved
2 - Normal multiuser mode - default mode
3-9 - Free to be defined by the administrator

Default runlevels

AIX - 2
Arch Linux - 3
Cent OS - 3
Debian GNU/Linux - 2
Gentoo Linux - 3
Mandriva Linux - 5
Mac OS X - 3
Red Hat Linux / Fedora Core - 3 or 5
Slackware Linux - 3
Solaris - 3
SUSE Linux - 5
Ubuntu (Server and Desktop) - 2

FreeBSD, OpenBSD and NetBSD do not use the concept of runlevels.