Wikis are different than blogs in that they are more collaborative with many people writing and editing, while blogs are more personal with usually one writer or author. Wikis are constantly changing with anyone having access to it and adding to it or making changes. Blogs are mainly written posts with opinions from the blogger and comments from others when approved by the blogger.
When thinking about classroom use and assignments, I think that the wiki is better suited for communication with parents and students. I like to use it to communicate assignments, projects, and important links. If students are working on a group project, they can work together to add links and resources, as well as create a project collaboratively on a page within the wiki. The wiki also offers a great place to showcase student work and classroom photos! The blog, on the other hand, seems to lend itself better to discussions. Creating a blog for students to discuss books/novels that are read is a great way to get them communicating about the literary elements and themes presented.
An initial problem with the wiki is teaching students how to edit and create on the wiki. Once they have had enough practice with adding links and images and embedding video, it is important for the teacher to monitor what is happening on the wiki so that students are using it appropriately. Rules for use need to be established and discussed, with consequences for misuse in place. The history tab is a great way for teachers to keep an eye on what each student is contributing to the wiki.
The classroom wikis that the teachers at our school have created have been a wonderful tool for all involved: teachers, students, and parents! What an awesome Web 2.0 Technology!