There are two types of partitions, which are sometimes three. Disks with the traditional partition tables have some physical limitations, this is true whatever operating system you are using. This means, you can have only four primary partitions, and that’s it. Only in this context, “that’s it” means that was, until someone smart enough came up with the idea of extended partitions.
So now, besides creating primary partitions, you also have the option to create an extended partition, that takes up one of the four available “slots” in the partition table. This means, you can have up to three primary partitions (but you can have less, but not more), and one extended partition.
These extended partitions can then hold any number of logical partitions you like. So if you need more than four partitions, you can have then (e.g. 13), you can have three primary partitions, and the rest can sit inside the extended partition. (In other words, you can slice up your extended partitions to what appears like “sub-partitions”, only nobody calls them that.)
One thing you cannot do is format, or directly use the extended partition. It is only used to hold the logical partitions, which can be used just like any other, they will have the same letters (a, b, c, d), and numbers (1, 2, 3, 4), as primary partitions would.