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

See :