Grub, is the standard boot loader used by each and every Linux type operating system that I can think of. RHEL 6 uses what I guess we are now calling grub 1.o, since grub 2.0 has been released and in use by Fedora for the last few releases. You will also find that grub 2.0 has replaced grub 1.0 in RHEL 7. At some point I plan to explore grub 2 at lenght, but today is not that day (unless something strange happens before I go to bed tonight -- you never know).
grub
RHEL6 – Restore Grub on MBR
GRUB, which stands for the GRand Unified Bootloader is the default boot loader in Linux these days ( it replaced LILO). When your server boots, the system BIOS transfers control to the Master Boot Record of your first boot device which is where Grub is installed. If the removed, damaged, or overwritten, then you will not be able to boot, and in which case you will need to repair/reinstall grub.