Every hard disk has a partition table with space for four entries. An entry in the partition table can correspond to a primary partition or an extended partition. Only one extended partition entry is allowed, however.
A primary partition consists of a continuous range of cylinders (physical disk areas) assigned to a particular operating system. If you have only primary partitions, you are limited to four partitions per hard disk (because more will not fit in the partition table). This is why extended partitions are used. Extended partitions are also continuous ranges of disk cylinders, but an extended partition can be subdivided into logical partitions. Logical partitions do not require entries in the partition table. In other words, an extended partition is a container for logical partitions.
If you need more than four partitions, create an extended partition as soon as the fourth partition or earlier. This extended partition should span the entire remaining free cylinder range. Then create multiple logical partitions within the extended partition. The maximum number of logical partitions is 15 on SCSI disks and 59 on (E)IDE disks.
It does not matter which type of partitions are used for Linux. Primary and logical partitions both work fine.