GNU GRUB ("GRUB" for short) is a boot loader package from the GNU Project. GRUB is the main example of the Multiboot Specification, which allows a user to have more than one operating systems on their computer at once, and to choose which one to run when the computer starts. GRUB can be used to select from different kernel images available on a particular operating system's partitions, as well as to pass boot-time parameters to such kernels.