There are quite a few different ones. Mine is a little easier since I'm only allowing my family. When you start adding friends and then their friends and so on it become a little more difficult to keep people out or finding you.
There is also an option in there that makes it where when people search for your name nothing shows up unless they are already on your friends list.
If you want to be hidden you have to pick a group that you want to be able to view or post to your wall otherwise you have them, their friends and then their friends and so on. Its a never ending cycle when done that way, but since I only use it to talk to family which is very rare I can have my settings like that.
I also have the ability to post to my wall disabled. I don't think people are aware you can do that as well.
Most of my setting is set on (only you)