The answer is "Google Recaptcha".
CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart and Google's reCAPTCHA is the main CAPTCHA service on the Web. You've likely observed CAPTCHAs a million times on join pages over the Web; to isolate people from spam bots, a test will fly up soliciting you to unravel a photo from words or numbers, choose questions in a lattice of pictures, or simply click a checkbox. Presently, however, you will see CAPTCHAs less and less, not on account of Google is disposing of them but rather in light of the fact that Google is making them undetectable.