While primary memory refers to RAM and processor caches, secondary memory refers to memory that is not accessed directly by the CPU. This includes storage devices that store persistent data, such as HDDs, SSDs, flash drives, and optical media.
When the CPU retrieves data from secondary memory, it is first loaded into RAM, since RAM can be accessed hundreds of times faster than a persistent storage device. Secondary memory, however, offers far greater storage capacity than primary memory. Additionally, secondary memory is non-volatile, which means it does not require an electrical current to retain its data. Therefore, secondary memory is sometimes called "persistent storage" or "permanent memory."
Updated: September 12, 2013