A computer system is a set of hardware and software which processes data in a meaningful way. A relatively simple computer system is a PC. An example of a relatively complex computer system is the Internet.
Even the simplest computer is really 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 be difficult due to incompatibilities, sometimes between differing hardware and sometimes software.
Different computer systems are not to designed to interconnect with each other. They can be made 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 a 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 to be followed. These standards are normally created by international bodies such as the IETF or IEEE. If your system obeys all of the rules, it is said to comply with the standard.