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. 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 is tricky in the best of cases due to incompatibilities, sometimes between hardware and sometimes software. We can say that the natural inclination of computer systems is not to interconnect themselves. So you have to force them, you have to make them adhere to a set of rules and constraints (protocols), that 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 themselves, and become a bigger system of sorts.
This "outside view" comes usually in the form of a standard, that is, a document explaining all the rules to be followed. If your system obeys with all the rules, it can be said to comply with the standard.