A blog is for [distributing information] to an involved audience by and authoritative figure. Blogs are best suited to providing announcements, news, experience accounts or expert opinions on the subject at hand.
Blogs have a single author or a small list of privileged authors. With a blog, authors are more important than the commenters.
Why use a blog as opposed to a forum or a wiki?
A blog is appropriate when you want to talk about experience, inform of news or give information from authoritative authors. In a forum, the respondents are at least as important as this the askers. In a wiki, you are recording information in articles for people to use. In a gallery, you're providing files to view or download.
How do I create one?
Blogs are created in the desired group or subgroup using the pencil icon to access the Manage > Manage Applications contextual panel. Here, you can create each type of application.