/etc/inittab

The inittab file describes which processes are started at bootup and during normal operation (e.g. /etc/init.d/boot, /etc/init.d/rc, gettys...). Init(8) distinguishes multiple runlevels, each of which can have its own set of processes that are started. Valid runlevels are 0-6 plus A, B, and C for ondemand entries. An entry in the inittab file has the following format:
id:runlevels:action:process

# inittab for linux
id:1:initdefault:
rc::bootwait:/etc/rc
1:1:respawn:/etc/getty 9600 tty1
2:1:respawn:/etc/getty 9600 tty2
3:1:respawn:/etc/getty 9600 tty3
4:1:respawn:/etc/getty 9600 tty4
This inittab file executes /etc/rc during boot and starts gettys on tty1-tty4.

init: The kernel starts PID 1 (init).
init: The init process reads the /etc/inittab and /etc/default/init and follows the instructions in those files.

what is a runlevel? the point at which the system is entered. Runlevel 1 is the most basic configuration (simple single user access using an text interface), while runlevel 5 is the most advanced (multi-user, networking, and a GUI front end). Runlevels 0 and 6 are used for halting and rebooting the system.
>init 6 This will reboot the system.
/etc/rc.d/rc.local file. This script file is run once, before all other scripts have run but before the logon prompt appears.
When the system is shut down, there is another symlink in the /etc/rc0.d and /etc/rc6.d directories (halt and reboot, respectively) that starts with a K instead of an S, which tells init to shut down the process.

Quick:
When you boot your computer, the first thing that it will do is load the bootloader -- either GRUB or LILO in most cases. The bootloader will then load the Linux kernel -- the core operating system. Next, a process called init starts. This process reads the file /etc/inittab to establish the runlevel to use. The runlevel is the start mode for the computer.
Once init knows the runlevel it will look for the appropriate files or directories as defined in /etc/initab.
Init will then either run the script files defined by /etc/initab, or run the script files in the directories defined by /etc/initab (depending on the set up of your system).
Finally, init will present you with the logon mode that you've selected.