Renting space to ad networks, like Google Adsense which choose the ads that are shown on your site, and affiliate links to products that you presell people on with reviews, comparisons or how-to articles.
Building up the site and continuing to add regular new quality content, at least twice a week is essential toward getting a decent amount of traffic, which usually comes from search engines, so you have to learn how to do SEO and get links from other sites. It takes a serious commitment of time to build up a money earning blog.
Google's Adsense is most commonly used to earn some supplemental income, some describe it as paying the electric bill. To earn their minimum payout level of $100 you will probably need tens of thousands of visitors, some blogs never reach the payout level.
Successful blogs can also be paid directly for writing posts endorsing a product.