Complex Systems are an interdisciplinary subject of research and are used as the theoretical framework in a wide range of experimental fields such as economic and medical disciplines, as well as in mathematical and computer science studies.
A complex system is composed by a large number of elements that interact with other.
The microscopic interaction among the elements gives rise to a global behavior of the system that can be very different, and at times even counterintuitive, with respect to the evolution rules of the single parts.
The simulated system does not necessarily reproduce the real one in all the details. Indeed, it is rather a synthetic representation of the real world that portraits a subset of the real observables that are supposed to be relevant for the description of the process under investigation.
That is why in complex-systems research projects the most important task is to identify and select the relevant variables and the structural foundations of the system being simulated.