GNU GRUB (short for GNU GRand Unified Bootloader) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Multiboot Specification, which provides a user the choice to boot one of multiple operating systems installed on a computer or select a specific kernel configuration available on a particular operating system's partitions.
GNU GRUB was developed from a package called the Grand Unified Bootloader (a play on grand unified theory.[3]) It is predominantly used for Unix-like systems. The GNU operating system uses GNU GRUB as its boot loader, as do most Linux distributions. The Solaris Operating System has used GRUB as its boot loader on x86 systems starting with the Solaris 10 1/06 release.
http://en.wikipedia.org/wiki/GNU_GRUB
GNU GRUB is a Multiboot boot loader. It was derived from GRUB, the GRand Unified Bootloader, which was originally designed and implemented by Erich Stefan Boleyn.
Briefly, a 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 Linux). The kernel, in turn, initializes the rest of the operating system (e.g. GNU).
GRUB 2 has replaced what was formerly known as GRUB (i.e. version 0.9x), which has, in turn, become GRUB Legacy. Enhancements to GRUB are still being made, but the current released versions are quite usable for normal operation.
GRUB Legacy is no longer being developed. For the differences between GRUB Legacy and GRUB, see the current status on the wiki and the Grub Legacy Documentation.