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Nice Question

When the system boots up following sequence is usually followed, unless you modify its normal flow (which you can in linux)

1. BIOS

2. MBR (Master Boot Record)

3. LILO or GRUB

4. Kernel

5. Init

6. Runlevels

BIOS:  It stands for Basic Input/Output. It searches, loads and executes the boot loader program. It means, it checks for boot loader from floppy,cd-rom,usb,hard drive, etc. and once found loads it into memory and assigns control to it.

MBR: It stands for Master Boot Record. Its usually located in first sector of bootable disk (mentioned above). Its always less than 512bytes in size. It has 3 parts: primary boot loader information, partition table, and validation checks. It contains information about GRUB, so loads it into memory and executes GRUB boot loader

GRUB: It stands for Grand Unified Bootloader. It has Kernel images in it. There can be more than one, in such cases it asks user to select one else goes ahead with default image as mentioned in conf file. "/boot/grub/grub.conf". GRUB loads and executes kernel and initrd images

Kernel: It mounts the root file system as mentioned in grub.conf "root=". Kernel executes /sbin/inint program which is the first process to execute. initrd is used by kernel as temporary root file system, unless kernel is booted and real root file system is mounted.

Init: Looks at /etc/inittab file to decide linux run level. There are 6 run levels: 0-halt, 1-Single user, 2-Multiuser without NFS, 3-Full Multiuser, 4-unused, 5-X11, 6-reboot. Default run level is usually set to 3/5

Runlevel: When system is booting up, various services get started. Those are the runlevel programs, executed from run level directory as defined in run level. Under /etc/rc.d/rc*.d or /etc/rc*.d there are various programs starting with S and K. S indicates to be ran while startup and K indicated to be ran while killing i.e. shutdown. Its followed by a number which indicated the sequence number of program when to be executed