Here's how to use it:
- Go to your Account Settings. It is in Account (top right) -> Account Settings.
- Here, find Account Security and click 'change'.
- In here, if you scroll down, it tells you about the recent sessions and also the currently active ones.
You can now click 'end activity' for each of the sessions. You are now secure.
Check out other tips and tricks in the archive. I have a number of Facebook tricks in there. You can learn how to check out your account from others' eyes, clean up your FB account and Profile, or to cancel a friend request you sent in error.