Runlevel 0 is reserved for the “shutdown” phase. Entering init 0 from the shell prompt will shutdown the system and usually power off the machine.
Runlevel 1 is usually for very basic commands. This is the equivalent to “safe mode” used by Windows. This level is usually only used to asses repairs or maintenance to the system. This is a single-user mode and does not allow other users to login to the machine.
Runlevel 2 is used to start most of the machines services. However, it does not start the network file sharing service (SMB, NFS). This will allows multiple users to login to the machine.
Runlevel 3 is commonly used by servers. This loads all services except the X windows system. This means the system will boot to the equivalent of DOS. No GUIs (KDE, Gnome) will start. This level allows multiple users to login to the machine.
Runlevel 4 is usually a “custom” level. By default it will start a few more services than level 3. This level is usually only used under special circumstances.
Runlevel 5 is everything! This will start any GUIs, extra services for printing, and 3rd party services. Full multi-users support also. This runlevel is generally used on by workstations.
Runlevel 6 is reserved for “reboot” only. Be carefully when running this command. Once you have entered init 6, there is no stopping it!
S, s – Single user mode. Doesn’t require properly formated /etc/inittab. Filesystems required for basic system operation are mounted.
0 – Go into firmware (sparc)
1 – System Administrator mode. All local filesystems are mounted. Small set of essential system processes are running. Also a single user mode.
2 – Put the system in multi-user mode. All multi-user environment terminal processes and daemons are spawned.
3 – Extend multi-user mode by making local resources available over the network.
4 -Is available to be defined as an alternative multi-user environment configuration. It is not necessary for system operation and is usually not used.
5 – Shut the machine down so that it is safe to remove the power. Have the machine remove power, if possible.
6 – Reboot
Notice Runlevel 5… so don’t mix them up 😉