I found this answer from Mark Berthelemy, who will be talking at out e-learning conference on October 30th & 31st.
Someone asked: "What's the difference between a blog, a forum and a conference? - They all look the same to me!"
I'm afraid there's no simple answer to that one. Much depends on the software you are using and the functionality it offers, but it also depends on how you choose to use that software.
My feeling is that a blog is centred on an individual (but it could be a group of individuals) who post an idea for dissemination amongst a wider audience. That wider audience can't post initial ideas, but they can comment on those ideas. Blogs tend to work really well to publicise ideas and as learning journals.
A forum is usually more "democratic" in that anyone can post a message. So they tend to be used more for groups learning together in a social constructivist model.
We had a long discussion about "blogs, forums & the nature of discussion" over on the Moodle forums a couple of years ago. You might find that useful, there's a lot of meat in it: http://moodle.org/mod/forum/discuss.php?d=27338 (login as guest if you haven't got a Moodle.org id yet).
Keywords: ast, blog, discussion, forum, norfolk