BootProcess > init
Init is the first program to run after your system is booted, and continues to run as process number 1 until your system halts. Init's job is to start other programs that are essential to the operation of your system. All processes are descended from init. For more information, see the manual page init(8).
The system initialization process is handled by the init daemon.
The ?/etc/inittab configuration contains tells init what to do. Especially it contains the lines :
id:2:initdefault: si::sysinit:/etc/init.d/rcS l2:2:wait:/etc/init.d/rc 2
Which causes the files /etc/init.d/rcS to run initialization scripts (in /etc/rcS.d/*), then scripts for requested runlevel (in /etc/rcS.[0-9]/*).