The MBR is sector 0, which is the first logical sector of a hard disk drive. It contains the first stage bootstrap code of a computer system. Because a standard sector only has 512 bytes, the MBR only has that much code space to specify the logic to complete the next stage of booting up a system.
Depending on the system, an MBR may contain OS (operating system) specific boot code, or the code to perform a multi-boot selection.
For a more detailed treatment, visit http://en.wikipedia.org/wiki/Master_boot_record.