What are primary and secondary storage devices?

Primary storage devices are non permanent, non removable mediums with a high speed access from the CPU. For example the RAM and CPU Cache are primary storage devices.

The secondary storage device are simply non-volatile mediums such as hard disk or SSD. Normally we talk about secondary storage device if no human interaction is mandatory to get access from it. For example a CD-RW or a USB dongle should be considered as an offline storage.