Content-control software, also known as censorware or web filtering software, is a term for software designed and optimized for controlling what content is permitted to a reader, especially when it is used to restrict material delivered over the Web. Content-control software determines what content will be available on a particular machine or network; the motive is often to prevent persons from viewing content which the computer's owner(s) or other authorities may consider objectionable; when imposed without the consent of the user, content control can constitute censorship.
From : http://en.wikipedia.org/wiki/Content-control_software