The document provides an overview of social networking sites (SNSs), including a definition, history, and scholarly context. It defines SNSs as web services that allow users to create public profiles, identify other users they are connected to, and view their own and others' connections. The history discusses early sites in the late 1990s like SixDegrees and the growth of popular sites in the 2000s like Friendster, MySpace and Facebook. It also reviews scholarly research on topics like impression management, networks and structure using data from SNSs. The purpose is to provide context for articles in the special issue on analyzing SNSs using different methods.