Click on Blogs on the sidebar under Features. Select the New Post link, Enter a Title and whatever you are wanting to blog in the Body section. Click Post to save the changes and make it public.
Members can post comments to your blog and you will receive an email when someone posts to it.
A blog is a Web Log. Mostly used to post journal or diary events. Basically whatever you want to talk about. They tend to be more personal than forums. Just stay within our site Terms of Service. Take a look at other blogs to get an idea.