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Master Boot Record (MBR)
When you turn on your PC, the processor has to begin processing. However, your system memory is empty, and the processor doesn't have anything to execute, or really even know where it is. To ensure that the PC can always boot regardless of which BIOS is in the machine, chip makers and BIOS manufacturers arrange so that the processor, once turned on, always starts executing at the same place, FFFF0h. This is discussed in much more detail here.
In a similar manner, every hard disk must have a consistent "starting point" where key information is stored about the disk, such as how many partitions it has, what sort of partitions they are, etc. There also needs to be somewhere that the BIOS can load the initial boot program that starts the process of loading the operating system. The place where this information is stored is called the master boot record (MBR). It is also sometimes called the master boot sector or even just the boot sector. (Though the master boot sector should not be confused with volume boot sectors, which are different.)
The master boot record is always located at cylinder 0, head 0, and sector 1, the first sector on the disk (see here for more on these disk geometry terms). This is the consistent "starting point" that the disk always uses. When the BIOS boots the machine, it will look here for instructions and information on how to boot the disk and load the operating system. The master boot record contains the following structures:
Due to the great importance of the information stored in the master boot record, if it ever becomes damaged or corrupted in some way, serious data loss can be--in fact, often will be--the result. Since the master boot code is the first program executed when you turn on your PC, this is a favorite place for virus writers to target.