This chapter discusses the concept of trust and how trust is used and modeled in
online systems currently available on the Web or on the Internet. It starts by
describing the concept of information overload and introducing trust as a possible
and powerful way to deal with it. It then provides a classification of the systems that
currently use trust and, for each category, presents the most representative
examples. In these systems, trust is considered as the judgment expressed by one
user about another user, often directly and explicitly, sometime indirectly through
an evaluation of the artifacts produced by that user or her activity on the system.
We hence use the term “trust” to indicate different types of social relationships
between two users, such as friendship, appreciation and interest. These trust
relationships are used by the systems in order to infer some measure of importance
about the different users and influence their visibility on the system. We conclude
with an overview of the open and interesting challenges for online systems that use
and model trust information.