In the new version of Moodle, the 'Course Availability' setting has been renamed to 'Visible' and has been moved to the 'General' section in the course settings.
Here you can "hide" your course completely. It will not appear on any course listings, except for managers, course creators, teachers and any other users with the view hidden courses capability. Even if students try to access the course URL directly, they will not be allowed to enter.
The hide/show courses capability controls whether a user can hide a course.
Note: In earlier versions of Moodle, this setting was named 'Course availability'.
Hope this helps!