GRUB (GRand Unified Bootloader) is an advanced bootloader that is capable of booting multiple operating systems on a single machine. It can load *nix as well as other proprietary operating systems. The folks from the MS Windows platform are unfortunately ignorant about the concept of bootloaders. Proprietary operating systems like Windows often hide the background features of a system, like bootloaders, from the user.

With the help of a bootloader you can theoretically load hundreds of operating systems. Most familiar Linux distros currently ship with GRUB by default. In short, GRUB is what is displayed immediately after the BIOS. It enables a user to select which as the machine should boot from a list, by using the arrow keys. One of the biggest benefits of GRUB is that it is dynamically configurable. Lilo is another bootloader, which was once the default and has now been depreciated by most distros.