Web 2.0 refers to online networks that foster participation and user-generated content. It allows users to collaborate and share information through tools like Flickr, YouTube, blogs, and file sharing services. Key characteristics of Web 2.0 include ease of use, informal communication, and social knowledge construction. Blogs specifically can be used by teachers and students to share course information, have class discussions, and allow students to write about their interests. When using the web for learning, teachers face challenges in engaging students actively and critically evaluating online sources.