Real memory usage is the use made by programs of the computer's actual memory-storing hardware while virtual memory is constructed internally and used to balance demands real memory can't meet. Virtual memory works through prioritization of tasks and multitasking, juggling programs within a smaller subset of memory.
When a Linux operating system has programs running that exceed the capacity for storage provided by the memory chips in the physical computer, virtual memory is created. This virtual memory bridges the shortfall.
Virtual memory tells programs that enough memory exists. It then shuffles operations among programs so that more operations can occur at one time and in sequence.