http://www.debian.org/doc/manuals/reference/ - Debian Reference's runlevels section
The groups of processes or working modes of a Linux which are started by ?init are controlled by the runlevel. The runlevel is a digit from 0 to 6 or the letter S. Runlevels 0, 6 and S are reserved for shutdown, reboot and single user mode. Runlevel 1 is also single user mode.
I.e. Debian has seven runlevels (0-6).
0 (halt the system) 1 (single-user mode), 2 through 5 (multiuser modes), and 6 (reboot the system).
Each runlevel designates a different system configuration (/etc/rc[0-6S].d/) and allows access to different processes.
Your system starts with the runlevel specified in /etc/inittab (which can be overridden at boot time, with kernel parameter).