An entire site can be set to private. This is ideal for an Intranet of a large school or department.
Only Site Administrators are able to modify settings.
Login
Go to Settings > Reading : Site Visibility.
Having a private site will limit the access to the site/pages and any media files (images, documents and etc.) within the site. A user is required to login to view the private site and will be prompted to do so when accessing a media file directly. When a site is set as private, all media files are moved to a private folder thus changing the URL.
Keep in mind that these sites do not meet legal standards for security.
You can also make specific post/page private and media files private on an individual base.