There are seven steps to the boot-up sequence.
1) BIOS (basic input/output system) – executes the MBR where Boot Loader sits,
2) MBR- Master boot reads Kernel into memory,
3) GRUB (Grand Unified Bootloader) Kernel starts Init process,
4) Kernel – Kernel executes the /sbin/init program. Init reads inittab, executes rc.sysinit,
5) Init – the rc script than starts services to reach the default run level and
6) Run level programs – these programs are executed from /etc/rc.d/rc*.dl/