Ubuntu boots into runlevel 2 by default, which means it starts the system as normal and leaves you inside the X Window System looking at the graphical login prompt. It knows what runlevel 2 needs to load by looking in the
rc*.d directories in
/etc. Ubuntu contains directories for rc0.d through to rc5.d and rcS.d.
Assuming that the value is
rc script executes all the scripts under the
/etc/rc.1 directory and then launches the graphical login.
If Ubuntu is booted to runlevel 1, for example, scripts beginning with the letter K followed by scripts beginning with the letter S under the
/etc/rc1.d directory are then executed:
matthew@seymour:~$ ls /etc/rc1.d/K10jackd K20rsync K20vboxdrv K80cups S70pppd-dns ...