Summary

Virtual memory is a crucial concept in computer systems that allows computers to effectively manage memory and run multiple programs simultaneously. 

It acts as an extension of a computer's physical memory (RAM), making it appear as if the computer has more memory than it actually does. When a program is running, it needs to store data in memory. If there is not enough physical memory available, virtual memory uses part of the hard disk as temporary storage. It transfers data between RAM and the hard disk as needed, which enables larger programs to run smoothly without crashing due to insufficient memory. 

Virtual memory is necessary because it allows efficient use of resources, enables multitasking, and ensures that programs can run even when the physical RAM is limited.