A primary storage device is a component in your computer that stores information for a short period of time. A primary storage device should not be confused with a "primary storage drive," which would be a hard drive that holds the operating system in your computer, like "Local Disk (C:)."
RAM, or random access memory, stores information that is being processed and offloads it at a very fast rate to the motherboard of the computer so it can be sent to the northbridge, and then for use to the GUI (graphic user interface), which allows the user to access the information with the mouse, keyboard, monitor, etc.
The processor, also known as the CPU (central processing unit), processes information on your computer. In order to do this, it needs somewhere to store the memory, which in this case is the "cache memory." The cache memory transfers data at lightning fast speeds so it can be processed by the cores in the processor. The cache memory holds a lot less space than RAM, however. For example, a processor will usually have around 12 MB of cache memory, whereas RAM may have up to 4 GB per stick. However, the cache memory makes up for that in sheer speed. For instance, RAM will have a speed of 800MHz, while the cache memory can operate at 2.4 Ghz.
The processor registers are the smallest of all primary storage devices. Typically, they hold around 32 to 64 bits, which is good enough for very simple processes such as math calculations. However, the processor registers are the fastest primary storage devices as well. They are primarily used by the processor to handle calculations used to operate the programs. The larger processes involving software and operating system files are handled by the cache memory.