Answer: Memory hierarchy is the hierarchy that is created on the basis of the response time of different memories. The performance obtained by the memory helps in creating a computer storage space in distinguished form. The factors considered for the creating of the hierarchy structure are usually response time, storage capacity, complexity etc.
Usage of different kind of memories take place due to different kind of requirements from the system which cannot be fulfilled using one memory device.The requirement is based on saving time, decreasing complexity , improving performance etc.Example of requirements can be like some functions and files do not require much space , some might require quick accessing,etc.
Thus hierarchy of any particular system is in the form of fast to slow order from registers,cache memory, Random access memory(RAM) and secondary memory.