We often encounter with a picture during a registeration on a secured website which is a text written in modified form or underlined or designed in such a way that it can be easily understandable only by humans. It is generally called CAPTCHA(Completely Automated Public Turing Test To Tell Computers And Humans Apart).As Shown in the Image Below...
CAPTCHAs are used in attempts to prevent automated software from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure. CAPTCHAs can be
