Grand Unified BootLoader
When a computer is turned on, the computer's BIOS finds the primary bootable
device (usually the computer's hard disk) and loads the initial bootstrap program
from the master boot record (MBR), the first 512 bytes of the hard disk, and then
transfers control to this code.
When GRUB receives control, it presents an interface where the user can select
which operating system to boot. This normally takes the form of a graphical menu,
although if this is not available or the user wishes further control, GRUB has its
own command prompt, where the user can manually specify the boot parameters.
GRUB can also be set to automatically load a particular kernel after a timeout period
After user selects the OS from the menu presented by the GRUB or the set timeout
period, OS kernel is loaded into memory. The control is handed over to the OS Kernel