BIOS: The Basic Input/Output System performs integrity checks on memory and seeks instructions on the Master Boor Record (MBR) on the floppy drive or hard drive. The MBR points to the boot loader (GRUB or LILO).

BOOT LOADER: Boot loader will then ask for the OS label which will identify which kernel to run and where it is located (hard drive and partition specified). It also pass some parameters to the kernel, most important: the run-level.

KERNEL: The first thing the kernel does is to execute /sbin/init program. Init is the root/parent of all processes executing on Linux.

INIT: The init program reads for instructions the file: /etc/inittab.

INITTAB: The inittab file describes which scripts are started at bootup depending on the run-level.

RUN-LEVEL SCRIPTS: All the scripts pointed by inittab are collected in the directory: /etc/init.d/. Based on the appropriate run-level, scripts are executed to start various processes to run the system and make it functional.