is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk. MBRs are usually placed on storage devices intended for use with IBM PC-compatible systems.
The MBR may be used for one or more of the following:
Holding the partition table, which describes the four primary partitions of a storage device. In this context the boot sector may also be called a partition sector.
Bootstrapping an operating system. The BIOS built into a PC-compatible computer loads the MBR from the storage device and passes execution to machine code instructions at the beginning of the MBR.
Uniquely identifying individual disk media, with a 32-bit disk signature, even though it may never be used by the operating system.