If you have a particular passion for something, whether it's a hobby or an obsession, and you have something to say about it, blogging could be a profitable way to pour out your endless stream of thought. The key here, as with many other services on the Internet, is in selling advertising.
After starting up a personal blog, many writers sign up for ad services like Google AdSense, which post those familiar sponsored links you often see at the top and on the sides of Web sites. The more times your blog readers click on those ads, the more money you'll make through the ad service. This works fine if you're a casual blogger, and you may make some extra spending money. But if the blog is consistently interesting, well-written and really takes off, you may be approached by companies who want to reach your fan base with graphical advertising around your blog. Some of the more successful blogs, like I Can Has Cheezburger? and Boing Boing, have become pop-culture phenomena, and their creators have been able to quit their day jobs and blog full time because of the money they make from advertisers.