A captcha is an automated program which runs on the website/server to judge whether the input fed is by a human or any automated programs/scripts/bots. It is used to stop false/spam entries. It can be used anywhere where there is a need to take inputs from the users like registration forms, comments etc. Typically a captcha program displays a distorted image with alphanumeric characters or a simple mathematical question which humans can answer but automated programs cannot.
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