Web 2.0 refers to websites that emphasize user-generated content, usability, and interoperability. Key characteristics include social networks that allow information sharing, folksonomies that organize content with user-generated tags, and content that can be easily reused on different sites. Examples include Google Maps, YouTube, Facebook, and Wikipedia. Web 2.0 sites often use a "perpetual beta" model where they frequently release updates based on user feedback to continuously enhance features and fix issues.