http://www.dedoimedo.com/computers/grub.html

Useful tutorial on the old Grub but modern Linux has left that behind
and now uses Grub2 - link below...

http://www.dedoimedo.com/computers/grub-2.html

GRUB is the bootloader and it is the first thing that comes up when you
boot Linux. It takes the form of a menu with all of the OSes that are
installed as menu choices. It can be text only or graphical depending on
the distribution. You can edit GRUB (which stands for GRand Unified
Bootloader) either temporarily or permanently.

GRUB is installed with Linux. You need a bootloader. There are other
choices, but GRUB is the most common.

It is assumed that you have set-up a dual boot system. The boot screen
lists the Operating systems you have installed on your computer and it
is the screen that by using the arrow keys and enter key allows you to
choose what operating system you will be using.

If after installing Ubuntu or ??? distribution on your machine you fail
to get this screen on booting your computer. It is likely that the Linux
install was not successful. installing Grub is the last thing the Ubuntu
installer installs and it is dependent on a successful install. If the
install failed check the live CD with the check sum and try another install.

If you have linux installed and you are not seeing grub (but think you
should be) hitting escape during boot will usually bring it up. On
single installations, the timeout is often set to zero so you just get the
default grub menu option. If you don't find the info you are looking for in
one of those other links, there is a very good thread on ubuntu forums that
you can find through their search.