Web2.0 refers to a second generation of web development that facilitates communication, sharing, and collaboration online through tools like social media sites, wikis, blogs, and user-generated content. Key characteristics include user-generated content, open sharing of information, and continuous updates. Popular Web2.0 sites for social networking include Facebook, Twitter, YouTube, Flickr, and blogs. These tools allow users to easily share content, comment on others' content, and connect with other users online.