The extended partition causes a lot of confusion for people setting up new PCs/HDDs. The extended partition really functions as a "container" that holds additional logical partitions except for the first (primary) volume. The reason that this structure was used is that the original design, with its limit of four partitions, was too restrictive. The extended partition system allows you to have up to 24 logical partitions in a single system.
Why "only" 24? Well with A and B for floppy, you only have 24 letters left. DOS and Windows are limited are limited to single letter drive designators, like "C" or "D". I dont know of any, but there *might* be filiing systems that allow a different system.
So long as the master boot record is not damaged, all PRIMARY partitions are independent. If one gets damaged the others are safe.
Within the extended partition, the logical drives are stored in a linked structure. The extended partition's information is contained in the master partition table (since the extended partition is one of the four partitions stored in the master boot record). It contains a link to an extended partition table that describes the first logical partition for the disk. That table contains information about that first logical partition, and a link to the next extended partition table which describes the second logical partition on the disk, and so on. The extended partition tables are linked in a chain starting from the master partition table.
Extendend partitions are NOT independent. If the boot record of one of the partitions gets damaged ALL partitions in the chained link will go down and data will be lost.
Always use primary partitions, UNLESS you desperately need more than 4. But in such a case, backups become more important.
Summary
HDD
---> Primary
---> Primary
---> Primary
---> Extended ("container")
.......>> Logical
.......>> Logical
.......>> ...