GRUB = "GR
It is the bootloader developed by the Open source community (specifically the Gnu Project) to replace LILO.
GRUB performs the same function as the Windows boot code.
When you say "modify", I think you mean change the behavior--this is done with the configuration file: /boot/grub/menu.lst
The "booting" link below might help, and their are lots of threads here at LQ--just search using "grub"