Completely Automated Public Turing Test to Tell Computers and Humans Apart is more commonly referred to by its acronym, CAPTCHA.
It is a type of challenge-response test used in computing to determine whether a user is a human or a machine. Websites use CAPTCHA to prevent spam and automated extraction of data. Because computers are unable to distinguish letters and numbers within moving or distorted images, CAPTCHA typically requires a user to enter letters or numbers presented in a visually distorted field. It is also common to choose from a variety of images where you need to select a common theme.