Short for master boot record, MBR is also sometimes called the master boot block, master partition boot sector, and sector 0. The MBR is the first sector of the computer hard drive. It tells the computer how the hard drive is partitioned, and how to load the operating system.
The picture above is an example of a partitioned hard drive. In this hard drive example, the MBR is the first section the computer looks at after the BIOS (basic input/output system) hands control to the first bootable drive. Unlike the VBR (volume boot record), 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 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.