Operating System Design/Physical Memory
Physical memory refers to the actual RAM of the system, which usually takes the form of cards (DIMMs) attached onto the motherboard. Also called primary memory, it is the only storage type directly accessibly to the CPU and holds the instructions of programs to execute. Physical memory is linearly addressable; memory addresses increase in a linear fashion and each byte is directly addressable.
Virtual memory adds a layer of abstraction over the physical memory, and offers many benefits such as the ability to maintain separate address spaces (per process perhaps), and the ability to use physical memory as a large cache for the physical disk which makes memory appear limitless (upto 2^(bus width)) from the program/programmers perspective.