Unlike primary memory, secondary memory is not accessed directly by the CPU. Instead, data accessed from secondary memory is first loaded into RAM and is then sent to the processor. The RAM plays an important intermediate role, since it provides much faster data access speeds than secondary memory. By loading software programs and files into primary memory, computers can process data much more quickly.
While secondary memory is much slower than primary memory, it typically offers far greater storage capacity. For example, a computer may have a one terabyte hard drive, but only 16 gigabytes of RAM. That means the computer has roughly 64 times more secondary memory than primary memory. Additionally, secondary memory is non-volatile, meaning it retains its data with or without electrical power. RAM, on the other hand, is erased when a computer is shut down or restarted. Therefore, secondary memory is used to store "permanent data," such as the operating system, applications, and user files.
NOTE: Secondary memory may also be called "secondary storage." However, this term is a bit more ambiguous, since internal storage devices are sometimes called "primary storage devices" as well.
Updated: December 8, 2012