Doporucuji precist Partition HOWTO.
http://tldp.org/HOWTO/Partition/index.html

Primary Partitions
The number of partitions on an Intel-based system was limited from the very beginning: The original partition table was installed as part of the boot sector and held space for only four partition entries. These partitions are now called primary partitions.

Logical Partitions
One primary partition of a hard drive may be subpartitioned. These are logical partitions. This effectively allows us to skirt the historical four partition limitation.
The primary partition used to house the logical partitions is called an extended partition and it has its own file system type (0x05). Unlike primary partitions, logical partitions must be contiguous. Each logical partition contains a pointer to the next logical partition, which implies that the number of logical partitions is unlimited. However, linux imposes limits on the total number of any type of partition on a drive, so this effectively limits the number of logical partitions. This is at most 15 partitions total on an SCSI disk and 63 total on an IDE disk.