What is Primary, Extended and Logical Volumes

What is the need of Extended and Logical Partition

The original partitioning scheme for PC hard disks allowed only four partitions. This quickly turned out to be too little in real life, partly because some people want more than four operating systems (Linux, MS-DOS, OS/2, Minix, FreeBSD, NetBSD, or Windows/NT, to name a few), but primarily because sometimes it is a good idea to have several partitions for one operating system. For example, swap space is usually best put in its own partition for Linux instead of in the main Linux partition for reasons of speed.

To overcome this design problem, extended partitions were invented. This trick allows partitioning a primary partition into sub-partitions. The primary partition thus subdivided is the extended partition; the sub-partitions are logical partitions. They behave like primary partitions, but are created differently. There is no speed difference between them. By using an extended partition you can now have up to 15 partitions per disk.

The extended partitions is a way to get around the fact you can only have four primary partitions on a drive. You can put lots of logical partitions inside it

You will never see hda4 mounted, just hda5 and hda6, in this case. 

Note: 

Linux numbers primary partitions 1-4, logical partitions start at 5 and up, even if there are less than 4 primary partitions.

On an IDE drive you can have up to 63 partitions, 3 primary and 60 logical ( contained in one extended partition )

What is partition table

A partition table is a 64-byte data structure that provides basic information for a computer's operating system about the division of the hard disk drive (HDD).

Partition table is part of the master boot record (MBR), which is a small program that is executed when a computer boots (i.e., starts up) in order to find the operating system and load it into memory. 

A partition is a division of a HDD into logically independent sections. Primary partitions are the first four partitions on a HDD.

The partition table begins at the hexadecimal (i.e., base 16) position 0x1BE in the boot sector. It contains four entries, each of which is 16 bytes in length, one for each partition.

The partition table entry for each partition consists of six items: the active flag, with 0x00 for off and 0x80 for on (one byte); the starting head, cylinder and sector (three bytes); the filesystem descriptor (one byte); the ending head, cylinder and sector (three bytes); the starting sector relative to the disk beginning (four bytes); and the number of sectors in the partition (four bytes).

Where MBR and Partition table are stored

The MBR, and thus the partition table, is stored in the boot sector, which is the first physical sector of a HDD. A sector is a segment of a track on a magnetic disk (i.e., a floppy disk or a platter in a HDD).

A track is any of the concentric circles on the magnetic media on a disk or platter over which one magnetic head (i.e., a device used for reading and writing data on the disk) passes while the head is stationary but the disk is spinning. A platter is a thin, high-precision aluminum or glass disk that is coated on both sides with a high-sensitivity magnetic material and which is used by a HDD to store data.

The MBR reads the partition table to determine which partition is the active partition. The active partition is the partition that contains the operating system that a computer attempts to load into memory by default when it is booted or rebooted.