Web 2.0 refers to web applications that enable sharing and collaboration through technologies like social media sites, wikis, blogs and folksonomies. It allows users to upload and share content, access information through browsers, and own their own data. Key characteristics include openness, freedom for users to contribute, and collective intelligence through participation. Web 2.0 uses technologies like Ajax and JavaScript to make pages function more like desktop applications and update dynamically based on new fetched data. It has applications in various sectors like academia, business, medicine and government.