Master Boot Record
Most PCs today use firmware called the BIOS. The BIOS is run when the computer first starts up where it initializes system devices. After system devices have been initialized the BIOS then launches a program on the Master Boot Record (MBR). The MBR is the first sector (first 512 bytes) of a storage device (hard disk, CD/DVD drive, USB drive...). The BIOS launches the program on the MBR of the first recognized BIOS boot device on your computer. The program (also known as a bootloader) reads the partition table also on the MBR and is then able to boot the operating system(s). Common bootloaders in Linux are GRUB and LILO.
Because the MBR is located physically on the disk can be saved and later recovered.
To save the MBR:
dd if=/dev/hda of=/path/mbr-backup bs=512 count=1
Restore the MBR:
dd if=/path/mbr-backup of=/dev/hda bs=512 count=1
To erase the MBR (may be useful if you have to do a full reinstall of another operating system):
dd if=/dev/zero of=/dev/hda bs=446 count=1