What is an extended partition? Why do I need to use it to install Ubuntu if I already have four partitions that are factory installed?
There is a limit on the number of primary partition on HDD. The reason is the structure of MBR (Master Boot Record). MBR has only 64 bytes to store the partition table. One partition's information will take 16 bytes. So apply your mathematics here and you will find only 4 Primary partitions are possible in HDD. If you create all the 4 primary partition and still you have some free space in HDD, it will become unusable.
To overcome the limit of four partition, you can create an Extended partition in which you can create Logical partitions. And remember Extended partition itself is a primary partition. In this way we can create more than 4 partitions.
So basically it is Extended partition which allows to create more than 4 partitions in a HDD. The extra partitions which we simply call Logical partition will always come under Extended partition.
I've found following links so helpful, hope you would like it:
However, most new computers use the newer GUID Partition Table (GPT) system instead. GPT supports up to 128 partitions by default (and that value can be raised, if necessary) and does not distinguish between primary, extended, and logical partitions.
Thanks to Rod Smith for this quite useful information! :)