Short for Master Boot Record, MBR is also sometimes referred to as the master boot block, master partition boot sector, and sector 0. The MBR is the first sector of the computer hard drive that tells the computer how to load the operating system and how the hard drive is partitioned.
The MBR contains a small amount of executable code called the master boot code, the disk signature, and the partition table for the disk. At the end of the MBR is a 2-byte structure called a signature word or end of sector marker, which is always set to 0x55AA. A signature word also marks the end of an extended boot record (EBR) and the boot sector.