It may be argued that all so-called "things" are actually systems. For example, a cup is a thing, but it is also a system for holding hot or cold liquid, or other material. The cup has a certain shape and a handle, it is made of non-porous material and so on, and it is put together in such a way as to provide a useful function. Describing this thing makes up information, and defines a system.
A system consists of components which are connected together in order to facilitate the flow of information, matter or energy. At arbitrary boundaries, a collection of interrelated components may be declared a system and may further be abstracted to be declared a component of a larger system. Systems enable "stuff" to be done. (It's tempting to say that systems enable "things" to be done - but that is confusing in this context.)
An open system[?] can be influenced by events outside of the declared boundaries of a system.
A closed system[?] is self-contained: outside events can have no influence upon the system.
Dynamic systems have components and/or flows that change over time.
A system could also be a method or an algorithm. Again, an example will illustrate: There are systems of counting, as with Roman numerals , and various systems for filing papers, or catalogues, and various library systems, of which the Dewey Decimal System is an example. This still fits with the definition of components which are connected together (in this case in order to facilitate the flow of information).
OD theorist Peter Senge developed the notion of organizations as systems in his book The Fifth Discipline.