runlevel

Standard runlevels
Standard runlevelsID Name Description
0 Halt Shuts down the system.
S Single-User Mode Does not configure network interfaces or start daemons.[1]
6 Reboot Reboots the system.

 Almost all systems use runlevel 1 for this purpose. This mode is intended to provide a safe environment to perform system maintenance. Originally this runlevel provided a single terminal (console) interface running a root login shell. The increasing trend towards physical access to the computer during the boot process has led to changes in this area

Linux

The Linux operating system can make use of runlevels through the programs of the sysvinit project. After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.

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Typical Linux runlevels

Most Linux distributions, in addition to the standard runlevels, define the following additional runlevels:
Typical Linux runlevelsID Name Description
1 Single-User Mode Does not: configure network interfaces, start daemons, or allow non-root logins.[2]
2 Multi-User Mode Does not: configure network interfaces or start daemons.[3]
3 Multi-User Mode with Networking Starts the system normally.[4]
4 Unused in special purpose
5 X11 As runlevel 3 + display manager.

 The additional behavior of this runlevel varies greatly. All distributions provide at least one virtual terminal. Some distributions start a login shell as the superuser; some require correctly entering the superuser's password first; others provide a login prompt, allowing any user access.
 In some cases, runlevels 2 and 3 function identically; offering a Multi-User Mode with Networking.

SUSE Linux

SUSE uses a similar setup to Red Hat:
SUSE Linux runlevelsID Description
0 Halt
1 Single
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


The services that run under a specific runlevel can be modified with YaST | System Services (runlevel) or with chkconfig command like the Red Hat based distributions.

http://en.wikipedia.org/wiki/Run_level