GRUB

GRUB is the boot loader that takes over and when loaded a small bit of code (what it calls stage 1) is executed on MBR.

The next stage of GRUB code allows it to access linux file systems, and it uses that ability to read and load configuration files that tells it was OS it can boot, where they are and what options to pass them.

in the case of linux, this will include a number of different kernel versions on the disk and often includes special rescue modes that can help with troubleshooting.

usually the configuration file also describes some kind of menu you can use to see and edit all of your boot options.

GRUB will display a splash screen, sometimes with graphics and a countdown.

You can choose from a list of operating systems to boot from

GRUB also allows you to view and edit specific boot-time settings that can be handy during troubleshooting (single user mode)