A hard disk (IDE) may be divided into a set of partitions.  A partition consists of a contiguous group of sectors.  A partition is a logical organization imposed upon a physical disk drive:  software enforces the behavior.
 

Partitions have the following properties:

• a name
• defined limits (alternatively, size and location)
• file system type
• access rights
• containment (programs may not normally violate the boundaries)

Originally, there were only primary (physical) partitions: four of them.  Each primary partition begins with a secondary boot sector.  The definitions of a hard disk's primary partitions are kept in the Master Boot Record.

In order to increase the number of partitions and maintain backward compatibility, one of the primary partitions is now able to assume the role of an extended partition.  An extended partition may be further segmented into logical partitions.  A logical partition has no secondary boot sector.  The definitions of the logical partitions are kept in the extended partition, not the MBR.

This limits a hard disk to four distinct bootable operating systems.