Original: GRUB is the reference implementation of the Multiboot Specification , which provides a user the choice to boot one of multiple operating systems installed on a computer , or select a specific kernel configuration available on a particular operating system 's partitions .
Simplified: 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 .