Private WP will make sure people can only read your blog after they log in. Already logged in users will see no difference.
Users who are logged out will get the login page, and only that.
Use this for private "intranet style" blogs, or closed community sites.
Note that the plugin offers no options. Activate it to turn it on, deactivate it to turn it off!
If you also use Windows Live Writer, be sure to check out
WLW Login.