When the internet first began, most of the pages were static and didn’t change unless someone manually edited the HTML. Recently, more and more sites have become dynamic. Blogs, Forums, Social Networks, and Wikis have contributed to a movement on the internet dubbed “Web 2.0”. Basically, Web 2.0 refers to website that allow regular users to read and write content to the web.
Forums were the first big step. Forums just let people post questions, answer questions, and share ideas with others. These have been around since the late 1980s when people did not have access to the internet.
Blogs are newer than forums. A blog is simply a self-publication for the web. You can create a blog for any topic you can think of. The blog writers create content and many blogs supply a comments section for feedback, so users can read the blog, then write their ideas.
Wikis, such as Wikipedia, are collections of content. A user can read, edit, and create articles. Then that information is out on the web for anyone to read. Wikipedia is the most popular wiki containing millions of articles on a variety of subjects. Because anyone can edit or create content, the content is almost guaranteed to be up to date.
Social Networks provide a way for real world and internet friends to communicate online very easily. These sites boil down to the simple fact that a user has a set of friends and can send them messages as well as having a profile page. Some sites serve a particular purpose, like delicious is only for bookmarks.
I hope this clears up any confusion about what Web 2.0 is.