This article covers online communities, their evolution, and their social impact. It explores online communities from ARPANET's late 1960s launch to the present, including wikis, MMORPGs, and virtual worlds. Virtual communities have commercial potential, but they must be built and maintained through a common identity, strong leadership, and continual participation. It also emphasizes understanding online community dynamics by examining their psychological, sociological, and ethical components. User profiles, moderation, and lurking vs. active involvement are discussed. Online communities offer informational help and social connections, but they can pose privacy problems, false identities, and undesirable behavior. Case studies of successful and unsuccessful online communities offer practical advice, as do future developments like multiplayer online gaming. Understanding the complexity of online communities and their impact on the digital world begins with this article.