There are nearly a dozen legitimate (and a couple dozen illegitimate) reasons why you would report a certain Facebook user to the FB authority. Whether you're reporting a fake account, someone who's posting highly inappropriate content, or some jerk who has been harassing you, Facebook wants to know exactly why you have a problem with said user.
Facebook has always asked for a little bit of detail when you report an account, but the new menu is a cleaner, more inclusive module for reporting bad Facebook behavior. Before, many of the options were presented in a drop down format. The new menu seen below presents everything at once and also include a couple of new options like "This Timeline doesn't represent a real person - Business of organization."
You are now allowed to select multiple reasons for reporting someone and once you continue on from that prompt, you'll be asked to fill out additional problems in a text field.
Everything you see in the "Report Box" is there because it's a violation of Facebook's Statement of Rights and Responsibilities. On Facebook, as will many social networks, the onus is on the user to make sure that everyone is playing by the rules. Not only does the ability to report/block users help Facebook in running a site that adheres to the rules, but it also gives many users an extra level of security.
On the topic of reporting stuff, Facebook just announced some improvements to mobile security as well - one of which is the ability to report stories and mark them as spam on your mobile device.[Via Inside Facebook]