"Defferent Run Levels "

Run Level is considered as a System state.In Linux Operating systems there are 7 Run Levels [0-6]

Run level 0 =power off

Run level 1 =Single user mode

Run level 2 =Network without NFS

Run level 3 =Text mode (multiUser)

Run level 4 =Not defined

Run level 5 =X11 (graphical interface,its our default)

Run level 6 =Reboot

here are Run level related commands:
[ls /etc/rc.d/] ---to see rc directory for individual run level
[ls /etc/rc.d/rc.5.d] ----to see run level 5 contents.we can see other run level contents replacing rc.0,rc.1,rc.2 etc.

All the run level files are stored in /etc/init.d