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GRUB: GRand Unified Boot loader
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First, what exactly is GRUB?
GRUB is a boot loader, which means it passes control of the boot process from the Power-On Self Test (POST) to the kernel of your GNU/Linux distribution.
GRUB works in a modular, layered fashion so that any unneeded modules are not loaded. Not only does this reduce execution time, but it saves valuable resources when running from removable media. GRUB optionally loads its configuration file at run/boot time, so you don’t have to type in commands manually each time.
GRUB has mainly 2 stages(and one intermediate stage).
stage1 one is installed in MBR ,stage2 in the /boot partition
stage1 just loads the stage2
stage2 is the core boot loader and gives the menu to select the os
stage 1.5 is an intermediate stage
stage 1.5 comes into picture where stage1 cannot directly load stage 2
Here stage1 loads stage1.5 and stage1.5 loads stage2
The command-line option is still available in case there is an error in your configuration file.
Alternatives to GRUB include LILO, syslinux and isolinux.
The benefit of GRUB is that it will work in many different types of boot devices, but you only need to learn one set of menu commands. In addition, GRUB can work on other forms of bootable storage, such as CD-R/W, USB flash memory keys, floppy disks, and even via a TFTP server with PXE ROM booting.
Special Effects:
GRUB directly access the ext2 file system.
No need to reinstall , for any change in boot loader configuration.
Only needs to reinstall if the /boot partition has changed
For more details:
http://www.gnu.org/software/grub/manual/grub.html
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