Facebook advertising lets you reach a very specific audience by integrating subtle ads into the user's news feed. If you're not familiar with Facebook, the news feed is the "home page" where a user views all the latest updates from friends, celebrities they follow and other popular stories.
Ads fit into the news feed seamlessly: They appear just as another post or story, with the only difference being the small grey text that reads "Suggested Post" and "Sponsored" next to your ad. You can also have your ads show up in the right-hand sidebar: