CAPTCHA is an automated system which tests a visitor to a website to verify that they are human.
A CAPTCHA requires the person submitting information to complete a test, usually a visual one, that only a human could complete. This could involve showing a picture of text, distorted enough that a human can recognise it but a machine couldn’t if scanning it automatically.
New alternatives to CAPTCHA simply ask humans to click a box, asking “are you a human?”
CAPTCHAs are used when there is a threat that an automated piece of software called a bot might submit information for spam purposes or to extract data from websites.