The Boot loader is one of the most important aspects/programs of all operating systems. Whether its GNU/Linux, MS Window or Mac OS, etc it must be present!. And it is also a very fragile one as well. That's why having a recovery disk is also very important otherwise, say you're a bit new to all these "stuff", then you might easily end up losing data on your hard disk drive for instance.
Anyhow, there are primarily two main boot loaders can be seen in computers, one type is called the primary boot loader which represents a small piece of code/instructions that lets you computer to boot or load the most primary software contents without problems (that are closely related to the actual hardware, such as sending display signals to the Monitor screen or checking your RAM for errors for instance). This is usually stored in a ROM (read only memory) or EEPROM chip. The best example for such a Boot loader is the BIOS on your computer's motherboard.
Although the functions of a primary boot loader can be quite limited because those ROM chips usually have smaller capacity + since every operating system has a way of its own doing things, it's almost impossible to add all the details into the ROM. Even if we put our OS boot information into it, yet it can be quite hard to update it whenever the OS developers want since updating a BIOS chip has is "risks".
As an answer to all these issues they came up with another kind of boot loaders that run after the BIOS|primary boot loader is executed called secondary boot loaders. They're usually stored in your HDD/USB, etc, a place called MBR (master boot record). Now, these type of boot loaders are OS specific (not always true because they do have the ability to boot into other OS boot loaders. Multi OS boot for instance) and are behind the perfect execution of your operating system into your RAM.
Although there are several types... yet in GNU/Linux there are primarily two main BL can be identified. One is called Lilo and the other is called GRUB. GRUB is the primary one (if I'm not mistaken) these days which is an official part of the GNU project as well.
There can be many occasions a boot loader can be "corrupted"... say due to a virus attack or while trying to install/reinstall a boot loader or can occur after using a disk repair app in GNU/Linux (yes, it's possible) . When it comes to GRUB, in GNU/Linux you can install/reinstall GRUB loader without any GUI, even if your GNU/Linux PC Laptop is un-bootable, as long as you have the Live CD, etc you can use a command called "grub-install".
But then again, when you have an excellent grub installer GUI, why the heck u wanna do that! :). Sorry to drag you along with all these "related" details (just kinda felt like writing em)... if you want to easily control your GRUB boot loader in Ubuntu, then there is an excellent, GTK written GUI called "Boot-Repair".
This "window" is not available for those of us who use the original, official Ubuntu LiveCD... thus once installed and run we should be prompted by the below one instead...
Main features...
*. Recover or Repair your Grub code in MBR when you can't even boot your OS.
*. Repair it within the OS (you know, even if you removed your boot loader, as long as you don't reboot your PC, you're gonna be fine, for a while :P)
*. Reinstall the loader.
*. Change the location of the GRUB installation (you know, installing it into an another HDD, etc).
*. Restore the original boot sector.
You can install the magnificent :D Boot-Repair in Ubuntu 11.04 Natty Narwhal, 11.10 Oneiric Ocelot, 12.04 Precise Pangolin, 10.10 and 10.04 by entering the below command in your Terminal.
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair
