If you mark your Watchlist, lists, ratings, etc. as private, we'll respect your setting and it will only be visible while logged into your IMDb account.
To quickly view what's been marked as private, we recommend viewing your Activity page while logged out of IMDb. Even if your lists and ratings are marked as private, roll-ups of that data may be aggregated and visible as badges or other recognition.
When your information is public, you get access to awesome features - like being able to compare your ratings against other IMDb users. See Col Needham's activity page for an example.