The following is an excerpt I cut from am Infinite Thinking Machine post for the sake of brevity:
Prior to the advent of tools like blogs and wikis, the internet was a read-only affair for educators. The web was a powerful resource for educators and students, but information only moved one way, from publishers to consumers, and it could not be edited by the consumer. Now, though, things are changing. It is now as easy to create and share online content as it is to consume it. Anyone can publish, share, and change information online. We have a read/write web, a two-way web… what many are calling the web 2.0.
Blogs, podcasts, wikis, and social bookmarking are all read/write web tools.