Where a data storage device has been partitioned with (what Microsoft terms) the MBR Partition Table scheme (i.e. the conventional IBM PC partitioning scheme), the master boot record contains the primary entries in the partition table. The partition table entries for other, secondary partitions are stored in Extended Boot Records, BSD disklabels, and Logical Disk Manager metadata partitions that are described by those primary entries.
By convention, there are exactly four primary partition table entries in the MBR Partition Table scheme, although some (rare) systems have been known to extend this to five, or to eight, entries.
Where a data storage device has been partitioned with the GUID Partition Table, the Master Boot Record does not contain the partition table (although it contains dummy data structures, a "protective MBR", to prevent utility programs that only understand the MBR Partition Table scheme from creating partitions on the disc) and is wholly unused as far as disc partitioning is concerned.
One may have an arbitrary number of partitions on a disk. However, the Master Boot Record (MBR, sector 0 of the disk) only holds descriptors for 4 partitions, called the primary partitions. Usually the BIOS can boot only from a primary partition. (Of course it can boot a boot loader that itself is able to access nonprimary partitions or other disks.) The descriptors for the remaining partitions, called logical partitions, are scattered along the disk in a linked list of partition table sectors, starting with the MBR.