A computer system consists of a set of hardware and software which processes data in a meaningful way. The personal computer or PC exemplifies a relatively simple computer system. The Internet exemplifies a relatively complex computer system.
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. Many computer systems can interconnect, that is, join to become a bigger system. Interconnecting computer systems can prove difficult due to incompatibilities, sometimes between differing hardware and sometimes between different software suites.
Designers of individual different computer systems do not necessarily aim to interconnect their product with any other system. But systems administrators can often configure even disparate computers to communicate using a set of rules and constraints known as protocols; these precisely define the "outside view" of the system. This outside view effectively defines the way one system connects with another. If two systems define the same "outside view", they can interconnect and become a larger computer system.
This "outside view" usually comes in the form of a standard, that is, a document explaining all of the rules a device or a program must follow. International bodies such as the IETF or IEEE normally set up or endorse such standards. If an individual system obeys all of the rules, systems designers say it "complies with" the standard.
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IBM, Compaq and Dell, Inc. exemplify PC-manufacturing companies or former PC-manufacturing companies.