I am often asked the difference between online forums and blogs. It is a very good question. First, both technologies share a lot in common. Both techniques engage the reader in a "conversation" and both involve some degree of community building. Both harness the power of the Internet to communicate in new ways, although forums and user groups have been around since the the Internet itself (and before) and blogs are relatively new, especially to business.
But there are some distinct differences between the two forms of "social" media.
User Groups and Online Forums
User groups or online forums were created specifically to build community on the Web around some specific interest or concern, for example, a user group comprised of Linux developpers, Mac Users, or Dreamweaver designers. But online forums are not just for technical folks, they can be used by any subset of users. For example, my father, a former editor of the Washington Post, is an active member of the Combat Correspondents online forum. There are also user groups for those who collect Beanie Babies, who share an interest in Star Trek or organic farming. The list is almost endless.
Originally these communities of interest could be found on dial-up, bulletin board services and on Internet newsgroups frequented by early adopters of computer technology. Later, Internet-based forums began to spring up around the World Wide Web. Today these forums cover the gambit from religion to politics and typically are referred to as online forums, user groups, message boards, discussion groups, or web forums.
Another distinct difference is that user groups are moderated, by one or even several moderators, whose role is to ensure that the group stays on topic, that good behavior is enforced and to help with any technical problems. The user group sets the agenda, and the users have a great deal of control. Online forums also often offer rich functionality, including greater security and authorization features, more formatting options and administrative controls. Because online forums are built to be moderated, the leader/s can control how the site is accessed, limit user posts, and even keep problem members from posting.
At the same time users have more options in an online forum, from private messaging to member profiles. Finally, one of the biggest differences is that user groups or online forums, can be read in order, in threaded conversations.
Blogs
Blogs, by comparison, are first and foremost, a publishing tool, a means to provide a voice to the individual blogger (or author). Where as an online community has many authors and represents many view points (often in conflict with each other), blogs offer their readers a single point-of-view, that of the author's.
Because blogs often include the ability for readers to comment on blog posts, a community often ensues. But this interaction is a byproduct of the dialogue between author and reader, and not the primary purpose of the blog. While readers can comment on a blog post, they don't generally set the conversation's agenda or subject. This differs greatly from the bilateral nature of online forums, where each member can be author or reader or both.
The tools used to publish blogs often include the functionality necessary for community-building found in Trackbacks and pings. These two techniques allow the author to comment on someone else's post and to begin a conversation. Blogs can be published as RSS feeds that provide automatic notification to subscribers when new posts are made. These feeds provide a mechanism for distributing or syndicating blog posts over the Internet. Then RSS readers, or aggregators, enable the reader to collect updates from many blogs and read them at his or her convenience. Thus, a community of interest can be quickly formed around people that subscribe to RSS feeds in common.
Blogs enable companies and project teams to manage their content or knowledge, while forums are more oriented toward answering individual queries. Blogs also make use of categories and allow the reader easily browse or search archived posts to find infomation on a particular topic or interest. And increasingly business blogs offer Google site search.
Today I see a melding of both technologies, where some online forums now offer RSS feeds and most blogs support community. But, while the two media share some aspects in common, they are different tools with distinct primary objectives. Each has a contribution to make to a company or enterprise. And which you use, in what fashion, depends on what you wish to accomplish. There is no reason you can't do both.
If you are interested in learning more about the two. Here is an excellent article on forums vs blogs and here is a thoughtful post.