A computer system is the combination of hardware and software. A typical computer system has memory and set of states that define the relationship between the system’s inputs and outputs. Although in earlier times the personal computer or PC was an example of a computer system, in practice a person considered a Computer Systems Engineer these days deals with a range of different computer architectures.
Even the simplest computer classifies as a computer system, because at least two components (hardware and software) have to work together. But the real meaning of “computer system” comes with interconnection. A peripheral is a piece of computer hardware that is added to a host computer in order to expand its abilities.
More specifically, the term is used to describe those devices that are optional in nature, as opposed to hardware that is either demanded or always required in principle. The term also tends to be applied to devices that are hooked up externally, typically through some form of computer bus like USB. Typical examples include joysticks, printers and scanners. Devices such as monitors and disk drives are not considered peripherals when they are not truly optional.