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 the hard drive is partitioned, and how to load the operating system.
In the picture above, is an example of a partitioned hard drive. In this case, the MBR is the first section of the hard drive the computer looks at after the BIOS hands control to the first bootable drive. Unlike the VBR, there is always only going to be a maximum of one MBR on a partitioned hard drive.
The MBR is also susceptible to boot sector viruses that can corrupt or remove the MBR, which can leave the hard drive unusable and prevent the computer from booting up. For example, the Stoned Empire Monkey virus is an example of an MBR virus.