(from wikipedia)
the term runlevel refers to the mode of operation in an OS that implements the Unix System V-style initialization.
runlevel defines the state of the machine after boot. Different runlevels are typically assinged to the:
The real thing you should know about this: the Linux Standard Base specification.
This is a general guide for what the run level ID's pair up too. In debian 2-5 are the same (full multi-user with console logins), RHEL is more secured but also a pain in the ass.
0 Halt 1 Single-User mode 2 Multi-user mode (sin networking) 3 Multi-user mode (with networking) 4 Not generally used 5 Multi-user mode (with x & console) 6 Reboot
So by editing /etc/inittab , you could in theory have a program enter runlevel 6 and reboot the system. >;]~