  Many real-world complex systems consist of a set of elementary units
connected by relationships of different kinds. All such systems are better
described in terms of multiplex networks, where the links at each layer
represent a different type of interaction between the same set of nodes, rather
than in terms of (single-layer) networks. In this paper we present a general
framework to describe and study multiplex networks, whose links are either
unweighted or weighted. In particular we propose a series of measures to
characterize the multiplexicity of the systems in terms of: i) basic node and
link properties such as the node degree, and the edge overlap and
reinforcement, ii) local properties such as the clustering coefficient and the
transitivity, iii) global properties related to the navigability of the
multiplex across the different layers. The measures we introduce are validated
on a genuine multiplex data set of Indonesian terrorists, where information
among 78 individuals are recorded with respect to mutual trust, common
operations, exchanged communications and business relationships.
