Date Added: Jun 2012
CAPTCHA stands for completely automated public Turing test to tell computer and human apart. Due to the enormous growth of Internet, security of web application has become a vital issue and many web applications facing a threat of Internet bot also known as Internet Robot is an automated script which executes over the web forms and wastes precious web space. CAPTCHA has become de facto standard for securing web applications from Internet Bot and almost all the registration web forms use this test. During a last decade many researchers has done a work on CAPTCHA systems. This paper is a collective survey of work done on CAPTCHA systems.