A sector on the computer floppy disk, hard drive, or other bootable disk drive that contains instructs the computer on how to boot from the drive. IBM PC disk drives have different types of boot sectors. There is the MBR (master boot record), which is the first sector of a portioned hard drive, and the VBR (volume boot record), which contains information 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, each partition contains its own VBR. When BIOS hands control over to the boot sectors, the MBR takes control and starts the boot process and refers to the VBR to point it to the partition information.