Search Engine Optimization specialist

Blog is an abbreviated version of "weblog," which is a term used to describe web sites that maintain an ongoing chronicle of information. A blog features diary-type commentary and links to articles on other Web sites, usually presented as a list of entries in reverse chronological order. Blogs range from the personal to the political, and can focus on one narrow subject or a whole range of subjects.
Many blogs focus on a particular topic, such as web design, home staging, sports, or mobile technology. Some are more eclectic, presenting links to all types of other sites. And others are more like personal journals, presenting the author's daily life and thoughts.

Blogs vs Forum
Forums are instrument used to communicate, interact and collaborate with others (examples include discussions, chat rooms, messaging windows, calendars). They save information posted on a particular topic (not in real time) for other people to see at any time, creating a discussion environment. Everything that gets posted can be read again and again.

Blogs were defined as collections of articles, ideas, news, facts, opinions or inspirations that are “posted” on the internet. They are usually structured, organized by category and are updated often, if not daily. The owner of the blog is able to control the content.

With both blogs and forums, you can post a comment and reply to other comments, thus developing discussions. In both cases, you can leave comments which may or may not be moderated and you may have to identify yourself with a name and email address.

weblogs and message boards *are* different -- different enough to happily exist together in the same online community web site. My conclusion is that online communities will use the two resources to fill two different roles. Their ability to fill independent niches will make the subtle differences between them make more sense.