It is a boot loader for PC/AT.
The standard boot loader for the Momonga Linux is LILO.
You have to update LILO manually in the case of update of kernel etc. However, with grub, at the time of booting up, it can change dynamically and is very convenient.
Operating Systems which can be booted by grub include:
Linux
Windows
MS-DOS
BeOS
FreeBSD
OpenBSD
GNU Hurd
Grub is distributed from http://www.gnu.org/software/grub/