Master Boot Record
The master boot record is the first sector on the harddisk.
It contains the "bootstrap." This is a program, which is called by the BIOS to load the actual operating system.
This sector also contains a description of how the harddisk is divided into separate areas (partitions). This description is listed in the so-called "partition table." It contains one entry for each partition. The entry lists the location of the partition on the harddisk and the "type" of this partition.
Due to historical reasons, this partition table can include only four entries.
The format of this master boot record (bootstrap / partition table) is permanently defined and is absolutely required by all operating systems.