If you have a particular passion for something, and you have a lot 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 being consistent (in this case blogging several times a week), selling advertising and using your blog as a platform to promote other businesses.
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 websites (like HowStuffWorks.com). 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 just want 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, which will give you more money.
You can also sign up for affiliate links. Affiliate links allow you to get a cut of any product sold via a link from your website to the advertiser's site. So, if you write a blog entry and hyperlink to a product in the entry, if a reader clicks on that link and buys the product, you get a percentage of the sale. Shareasale is just one place you can sign up for links.
But advertising can only take you so far. More money can be made by thinking of your blog as a springboard to other gigs. For instance, the blog Digital Photography School has a community of over 2,000,000 readers and makes money from advertising, affiliate links, e-book sales and photography courses [source: Sparring Mind].
A travel blogger may start out documenting the highlights (and lowlights) of their trips around the world. They buoy the text with video clips, lots of hashtags, and of course, appropriate shareable social media links. From that, they may decide to lead tours to places they have been, do speaking engagements or create e-books with travel tips and sell them from their sites. They may even lead workshops teaching others how to make money from a blog. (Nomadic Matt's and Adventurous Kate's blogs are good examples of these principles in action.)