MBR, or Master Boot Record, is a small program, that is executed when a computer boots up. Typically, the MBR resides on the first sector of the first Hard Disk. The program begins the boot process by looking up the Partition Table to determine which Partition to use for booting. It then transfers program control to the Boot Sector of that Partition, which continues the boot process. In MS-DOS and Windows systems, you can create the MBR with the FDISK /MBR command.
In the modern scene, the MBR can be programmed with a Boot Loader. The task of this program is not not only transfer control of the computer to the operating system, but to present a list of choices of how to boot, not just one, but potentially many Operating Systems.