Primary partition, also called primary disk partition, is a type of partitions same as extended partition and logical partition. Other types of partitions can not be divided in the primary partitions, so every primary partition is equal to a logical disk (At this point the primary partition and logical partition are similar to each other. But the primary partition is directly divided in disk, and logical partition must be built in extended partition).

Actually, in the earlier disk partition there is no concept of primary partition, extended partition and logical partition. The type of every partition is primary partition. As only 64 bytes are reserved for partition table and every partition takes up 16 bytes, only data of 4 partitions can be stored in Boot sector, which means that a physical disk can only be divided into 4 logical disks. But these 4 logical disks can not meet the actual demands in specific application. In order to create more logical disks for operating system, the extended partition and logical partition are introduced and the original type of partition is called primary partition.

Related information
1. A hard disk has 1-4 primary partitions. And it has 1 extended partition at most. And the total number of primary partition and extended partition must be less than 4. And it may have several logical partitions.
2. When the primary partition is divided, all of the left part can be divided into extended partitions usually. Otherwise, the space not divided will be wasted. Of course, MiniTool Partition Wizard can be applied to adjust capacity of primary partition and extended partition. The related help can be found in Resize Partition Help Center
3. Extended partition can not be applied directly unless it is divided into several logical partitions. And all logical partitions are a part of extended partition. Capacity of hard disk = capacity of primary partitions + capacity of extended partiton
Capacity of extended partition= capacity of all logical partitions
4. The logical disk composed of primary partitions and logical partitions are called Drive or Volume.
5. The activated primary partitions will become Boot Partition, and it will be considered as the first logical disk by operating system or mainboard (In DOS/Windows it will be identified as Drive C: or Local Disk C:). The important files (such as Boot Record, bboot.ini, ntldr, ntdetect.com) related to enablement of DOS/Windows must be located in Boot Partition.
6. The inactive primary partitions and extended partition can not be seen in DOS/Windows. But all partitions can be seen in Disk Management of Windows 200/Vista and other NT editions of kernel.