BIOS,MBR,Boot Sector
MBR refers to the first physical sector of the hard drive. MBR contains the disk signature and partition table.
Boot sector refers to the file system boot sector, which is the first sector in one of those partitions created on the disk. Each partition has a boot sector. Each hard disk has only one MBR.
With multiple hard disks' system, BIOS determines which dis to use as the "boot device". BIOS loads the first physical sector from the boot device--MBR. MBR loads the boot sector from Active Primary Partition.