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, initialises the rest of the operating system (e.g. SliTaz).

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.