  Multilayer networks represent systems in which there are several topological
levels each one representing one kind of interaction or interdependency between
the systems' elements. These networks have attracted a lot of attention
recently because their study allows considering different dynamical modes
concurrently. Here, we revise the main concepts and tools developed up to date.
Specifically, we focus on several metrics for multilayer network
characterization as well as on the spectral properties of the system, which
ultimately enable for the dynamical characterization of several critical
phenomena. The theoretical framework is also applied for description of
real-world multilayer systems.
