This information is summarized from the GNU/GRUB web site http://www.gnu.org/software/grub/#documentation and from the grub info pages (``info grub'') Many Linux distributions give you the option of using GRUB as you bootloader, during the install. You don't need all this information to use grub. However, if you need to dual boot to a proprietary OS, or need to recover from a damaged Master Boot Record, you may find this information valuable.
GNU GRUB is a Multiboot boot loader. It was derived from GRUB, GRand Unified Bootloader, which was originally designed and implemented by Erich Stefan Boleyn.
Briefly, boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel software (such as the Hurd or the Linux). The kernel, in turn, initializes the rest of the operating system (e.g. GNU).
GNU GRUB follows these requirements below:
One of the important features in GRUB is flexibility; GRUB understands filesystems and kernel executable formats, so you can load an arbitrary operating system the way you like, without recording the physical position of your kernel on the disk.
Thus you can load the kernel just by specifying its file name and the drive (and the partition) where the kernel resides. To let GRUB know the drive and the file name, you can either type in them manually via the command-line interface, or use the nice menu interface through which you can easily select which OS it boots. To allow you to customize the menu, GRUB will load a preexisting configuration file . Note that you can not only enter the command-line interface whenever you like, but also you can edit specific menu entries prior to using them.