What is the difference between "extended" partitions and "logical" partitions on my hard disk? What's the need for each? I am using Linux
Historically, hard drives were only been able to contain at most four partitions because of the originally defined format of the partition table. This is not specific to operating systems. You simply can't create more than four primary partitions under the original PC partition scheme (MBR partitions).
In order to circumvent this limit and still remain compatible with older systems, you can create an extended partition however. An extended partition can contain multiple logical partitions within it. This allows you to create more than four partitions in total, without having to change the format of the partition table.
This limitation and the concept of extended and logical partitions do not apply to other partitioning schemes such as GPT used by UEFI on recent PC.