Primary partition is used for OS booting, maximum of 4.
Extended partitions are extended from the primary partitions,
usually use for storage of other stuffs.
If you use Windows only, and has backed up your files in D or E or
whatever drive, that's the extended partition. These extended
partitions can be further split into logical ones, such as E, F, G,
H and so on. These extra logical partitions can be used to store
other stuffs as well.
Say C is Windows, D is for your backups, E is for your games, F is
for your music and video files.
If let's say you dual boot, C is Windows, D is Linux, E is Mac,
then C, D and E are your primary partitions.
Depending on how you set up your system, D could be your extended
partition for Windows backups, making E your primary partition for
Linux (technically no such things, because there's no A, B, C, D, E
or whatever naming in Linux).