Captcha is mainly used to prevent automated software from performing actions on behalf of actual humans. For example, while signing up for a new email account, you will come across a Captcha at the end of the sign-up form so as to ensure that the form is filled out only by a legitimate human and not by any of the automated software or a computer bot. The main goal of Captcha is to put forth a test which is simple and straight forward for any human to answer but for a computer, it is almost impossible to solve. In simple words, Captcha is the word verification test that you will come across the end of a sign-up form while signing up for Gmail or Yahoo account. Some Websites implement a totally different Captcha system to tell humans and computers apart. For example, a user is presented with 4 images in which 3 contains picture of animals and one contain a flower. The user is asked to select only those images which contain animals in them.
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