On a forum, everybody can start a topic. On a blog, only the owner of the blog can start a new topic. On an article directory, the article writer starts the topic.
On a forum, people ask questions hoping that someone will have the right answers for them. On blogs and article directories, the writers have the answers, hoping someone will ask the right questions.
An article directory is actually a moderated blog, with a lot of writers. Is moderated, because the owner of the article directory decides, in the end, which articles will be published.