A sector on a computer floppy disk, hard drive, or another bootable drive with instructions on how to boot from the drive. IBM PC disk drives have different types of boot sectors. An MBR (master boot record) is on the first sector of a partitioned hard drive with a VBR (volume boot record) at the beginning of each partition.
In the above image, is an example of what a partition hard drive may look like with MBR and VBR boot sectors. In this example, the drive has two partitions, and each partition contains a VBR. When BIOS hands control over to the boot sectors, the MBR takes control and starts the boot process and refers to the VBR for partition information.