The second part contains definitions and characteristics about social networks. “Social networks” is a broad term; any gathering of human beings for a reason is a social network, therefore, they can be found everywhere. This definition is addressed in this Thesis, but the instances of this concept used through this review are social networks as applications and services based on Web 2.0. The origin of these Web 2.0 social networks is presented, with the examples of Usenet, Classmates.com and Open Diary being the first ones. Some optimistic and pessimistic scenarios on the usefulness of social networks for working and educational environments are presented, as well as some studies about perceptions of people, employees and businesses on social networks.