About GRUB ? Red Hat GRand Unified Bootloader ?

Before linux is installed on a system. Special instructions must be Placed in
Boot Loader (Its a programme that exists on Systems Primary Hard Drive or other Media
that knows how to start the linux Kernel)used to Boot the system.Red Hat linux installation
Programme allows user to quickly and easily configure the boot loader in the
Primary hard dirve's Master Boot Record (MBR) to load the OS.

Define GRUB :

GRUB (GRand Unified Bootloader) is a programme that installs a Boot Loader to the MBR. This MBR
exists at the beginning sector of the disk.
The Grub menu is also command environmental means we can pass specific arguments as instructions.

The process of loading operating GRUB and OS involves several stages :

1)Load the primary boot Loader (stage 1):

It exists in very small space allocated for MBR ( < style="font-weight: bold;">2)Load the secondary boot Loader (stage 2):

Choosing of operating system. or give commands or modify commands

3)Load the Operating System (linux kernel on specified partition):

After receiving correct instructions for OS to start from command line or
Configuration file