Central Processing Unit:

A Central Processing Unit (CPU) in a computer system is an extremely fast device as compared to the main memory, but it can not work on its own. It depends on the main memory that sends data and instruction when required for processing.

 In other words, we can say that even memories with smaller size (i.e. primary memory) have very high access time (time taken by CPU to access a location in memory), which reduces the overall speed of a computer. 

Computer memory can also be categorized on the  basis of Volatile  and Non-Volatile characteristics. Non-volatile Memory is a type of computer memory which can retain the stored information even if the power is shut down. Examples of Non-Volatile Memory are Read-only memory (ROM), flash memory, optical discs etc. On the other hand, a Volatile memory loses its content when the power goes off. Random Access Memory (RAM), which is the most common form of Primary Storage, is a type of volatile memory. When the computer is shut down, everything contained in RAM is lost.