  The enormous increase of popularity and use of the WWW has led in the recent
years to important changes in the ways people communicate. An interesting
example of this fact is provided by the now very popular social annotation
systems, through which users annotate resources (such as web pages or digital
photographs) with text keywords dubbed tags. Understanding the rich emerging
structures resulting from the uncoordinated actions of users calls for an
interdisciplinary effort. In particular concepts borrowed from statistical
physics, such as random walks, and the complex networks framework, can
effectively contribute to the mathematical modeling of social annotation
systems. Here we show that the process of social annotation can be seen as a
collective but uncoordinated exploration of an underlying semantic space,
pictured as a graph, through a series of random walks. This modeling framework
reproduces several aspects, so far unexplained, of social annotation, among
which the peculiar growth of the size of the vocabulary used by the community
and its complex network structure that represents an externalization of
semantic structures grounded in cognition and typically hard to access.
