GRUB is the boot-loader (the software that starts or boots the computer to action when it starts up) used on Ubuntu and various other systems. The latest edition of GRUB is GRUB2. It features better maintainability and automation than it's predecessor. However, some people find GRUB Legacy to be easier to use.
Comparisons apart, GRUB2 can be reinstalled to the computer after it has been removed or deleted by this process:
Comparisons apart, GRUB2 can be reinstalled to the computer after it has been removed or deleted by this process: