CAPTCHA is the computer challenge-response test used to ensure that the response is made by a human and not another computer. The process usually involves a computer asking a user to complete a simple test which the computer is able to grade.
These tests are designed to be easy for a computer to generate but difficult for a computer to solve, but again easy for a human. They aren’t always easy for humans, though. If a correct solution is received, it can be presumed to have been entered by a human.
A common type of CAPTCHA requires the user to type letters and/or digits from a distorted image that appears on the screen. Such tests are commonly used to prevent unwanted internet bots from accessing websites, since a normal human can easily read a CAPTCHA, while the bot cannot process the image letters and therefore, cannot answer properly, or at all.