(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:

• single-user mode (root)
• multi-user mode without networking services started
• multi-user mode with networking services started
• system shutdown
• system reboot

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. >;]~