Network effects — where a product becomes more valuable as more people use it — have created the most valuable companies of the digital age. This article provides a taxonomy of network effects: direct (telephone), indirect (operating systems), two-sided (marketplaces), and data network effects (machine learning). It examines the dynamics of network effect businesses including tipping points, winner-take-most markets, multi-homing, and defensive moats. Case studies span from Metcalfe's original law to modern platform businesses.