Hard Disk : Partition Types

There are three kinds of partitions: primary, extended, and logical. Primary and extended
partitions are the main disk divisions; one hard disk may contain up to four primary
partitions, or three primary partitions and one extended partition. The extended partition
can then be further divided into any number of logical partitions.

Primary Partitions

A primary partition may contain an operating system along with any number of data files
(for example, program files or user files). Before an OS is installed, the primary partition
must be logically formatted with a file system compatible to the OS.
If you have multiple primary partitions on your hard disk, only one primary partition may
be visible and active at a time. The active partition is the partition from which an OS is
booted at computer startup. Primary partitions other than the active partition are hidden,
preventing their data from being accessed. Thus, the data in a primary partition can be
accessed (for all practical purposes) only by the OS installed on that partition.
If you plan to install more than one operating system on your hard disk, you probably
need to create multiple primary partitions; most operating systems can be booted only
from a primary partition.

Extended Partitions

The extended partition was invented as a way of getting around the arbitrary four-partition
limit. An extended partition is essentially a container in which you can further physically
divide your disk space by creating an unlimited number of logical partitions.
An extended partition does not directly hold data. You must create logical partitions
within the extended partition in order to store data. Once created, logical partitions must
be logically formatted, but each can use a different file system

Logical Partitions

Logical partitions can exist only within an extended partition and are meant to contain
only data files and OSs that can be booted from a logical partition (OS/2, Linux, and
Windows NT).
The illustration below shows a hard disk that contains four main partitions: three primary
partitions and one extended partition. The extended partition has been further divided into
two logical partitions.
Each primary partition has been formatted to use a different file system (FAT, NTFS, and
HPFS). The two logical partitions have both been formatted to use the FAT file system.