GRUB is one of the most common used bootloader on linux.
So, when using linux, and you’re a bit interesting in ‘what’s under the hood’, it’s the first thing you want to learn about.
When you turn on your computer, the bios will start the bootstrap procedure from the primary boot-device.
(If you’re planning to run a LiveCD, you might make that your CD/DVD-rom drive) 😉
A bootstrap is in fact nothing else than having a small program, so that this can load a bigger one.
That’s the thing GRUB does, in short!
What you should remember, is that it’s important, and you would not like to break it.
If you do, you’ll render your hardisk unbootable.
So the first thing you want to do, is back up your MBR!
Why not just GRUB?
The MBR is a 512-byte segment, the first sector, on your harddisk.
GRUB takes 446 bytes, the partition table takes 66 bytes
and the 2 remaining bytes are for a signature.
You might want to keep these 3 intact.