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CAPTCHA is an acronym for “Completely Automated Public Turing test to tell Computer and Human Apart”. A project of the Computer Science Department of Carnegic Mellon University (trade mark owned by Cernegic), CAPTCHA is a mechanism to tell humans and computers apart, automatically. In short, CAPTCHA is an application that can generate and grade tests that a human can pass, but are difficult (though not impossible, as we will see later) for a computer to pass.

The most common type of CAPTCHA is a challenge-response kind of a system. The application generates a distorted image of text characters, numbers, etc, that are difficult for a computer program to understand, but can be understood and read by a human. An example of CAPTCHA is the image below taken from wikipedia.org.