GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a user the choice to boot one of multiple operating systems installed on a computer if (dual booted) or select a specific kernel configuration available on a particular operating system's partitions.It loads up before the operating system
GNU GRUB was developed from a package called the Grand Unified Bootloader (a play on Grand Unified Theory). It is predominantly used for Unix-like systems.