Private or Public content
For any content that you post, you can decide what groups are able to see it. While you are editing or creating the content, click on >View/Edit permissions section and you'll see a bunch of roles. For each you get to choose whether that role can view edit or delete your work.
Going from most general (largest audience) to most specific (smallest)....
Anonymous users are people who browse the website but are not logged any - pretty much anyone in the outside world.