GRUB

GRUB, or GRand Unified Bootloader, is a free and open source bootloader for x86 computers. It is designed to easily allow the booting of multiple operating systems, such as Linux, Windows, or FreeBSD.

How it works
GRUB consists of several parts. The first part is a simple bootstrap code that the BIOS finds on the hard drive and launches. This code then loads the next part of GRUB, which scans the hard drive and displays a list of available operating systems. This list is generally found in the /boot/grub directory of the primary Linux/BSD installation, but can also be found elsewhere.

The menu.lst file
The menu.lst file is the default name for the menu and instructions for booting each operating system at boot. For a typical Linux installation, an entry will look something like: