A term used to describe a variety of Web-based platforms, applications and technologies that enable people to socially interact with one another online. Some examples of social media sites and applications include Facebook, YouTube, Del.icio.us, Twitter, Digg, blogs and other sites that have content based on user participation and user-generated content (UGC).
See “Social Media Tips for Online Marketing Plus 30 Social Media Sites” in the Did You Know…? section of Webopedia.