When you power on your computer, the first software that runs is a bootloader that invokes the computer's OS. phpGRUB, the GRand Unified Bootloader, is an integral part of many Linux systems.

Most Linux distributions that use GRUB come with it installed and ready to use. Many of the distributions that do not have GRUB installed by default have it available in their package systems; check there first before doing a manual installation. (from Linux.com)