Master Boot Record
Most PCs today use firmware called the BIOS (Basic Input/Output System). The BIOS is typically contained within a CMOS (Ceramic Metal Oxide Semiconductor) and executes upon system power-up, when it then 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 drive, solid state drive, CD/DVD drive, USB drive...). The BIOS launches the program on the MBR of the first recognized BIOS boot device on your computer. This program (also known as a bootloader) reads the partition table (also contained within the MBR) and eventually boots the operating system(s). Common GNU/Linux bootloaders include GRUB and LILO.
Backup and Restoration
Because the MBR is located on the disk it can be backed up and later recovered.
To backup 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