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 and allows access to different processes.
Your system starts with the runlevel specified in /etc/inittab
[http://www.debian.org/doc/manuals/reference/ch-system.en.html#s-custombootscripts wiki:?RunLevels Customizing].