Blogging is a way to share/communicate thoughts and ideas about a topic of interest.  It is collaborative in the sense that people who have access to the blog are encouraged to respond to another person's posts.  One way that a teacher can use a blog would be to have students review novels as though they are in a virtual book club.  Another way to use a blog would be to replace the class newsletter by creating a blog that communicates to parents and/or students about what is happening in the classroom.