A CAPTCHA is a type of challenge-response test used to ensure that the response is generated by a person. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are supposedly unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human.
A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen. But sometimes, you’re presented with some pretty crazy results: