Blog: A website where one person stands on a soapbox and gives his two-cents' worth on a variety of topics. Some blog "followers" might comment, and those comments are usually heavily moderated (usually pre-approved before they appear on the site). In summary: the blog owner is front-and-center on his blog.
Forum:
Message Board:
A website where hundreds of users post and discuss topics in a short of virtual coffee-shop. The owner(s) of the forum is "just another member" except he has more privileges than the other users. He probably will post a decent amount on his own forum, but his posts amount to less than 1% of the total forum content.
In summary: the forum owner just happens to own/run the forum. The forum is much more about the community that frequents it, rather than the owner himself.