A **complex system** is formed by many elements that interact locally, and, owing
to this interaction, an emergent behavior arise at the global scale.
Moreover, the emergence of these collective phenomena cannot be explained
by adding up the contribution of each isolated small unit:
complex systems cannot be understood with the principle of superposition. On the contrary,
we face scenarios in which different space and time scales are coupled
and the degrees of freedom compete between them, thereby resulting in
highly nonlinear scenarios that are hard to tackle.

A **network** is a mathematical object composed by a set of nodes and links.
Network science is devoted to the study of networks, in a broad sense: from
topological characterization to dynamical processes running on top of
them. Networks provide a very useful framework to study complex
systems: the individual units are the nodes, and the interactions
are denoted by the links.