Grand Unified Boot Loader (GRUB)

GRUB is an open-source bootloader that can directly or indirectly load a variety of operating systems.

There are now two versions of GRUB in common use.

GRUB 2.0 is the current version of GRUB. Most modern versions of Linux including Ubuntu and Fedora use this version.
GRUB 0.98 aka Legacy GRUB is the original version of GRUB. Red Hat and CentOS still use this version.