Grub
 

What does the Grub do exactly?

Stands for GRand Unified Bootloader. Bootloader ( like LILO ) for *nix systems. Can load all OSes I have heard of , plus some. Unlike LILO, it is filesystem aware and hence much more flexible, ie you can choose your kernel image and edit the boot command line at BOOT time.

You can obtain, configure and install grub from:
http://www.gnu.org/software/grub/