Google's reCAPTCHA is the leading CAPTCHA service (that's "Completely Automated Public Turing test to tell Computers and Humans Apart") on the Web. You've probably seen CAPTCHAs a million times on sign-up pages across the Web; to separate humans from spam bots, a challenge will pop up asking you to decipher a picture of words or numbers, pick out objects in a grid of pictures, or just click a checkbox. Now, though, you're going to be seeing CAPTCHAs less and less, not because Google is getting rid of them but because Google is making them invisible.
The old reCAPTCHA system was pretty easy—just a simple "I'm not a robot" checkbox would get people through your sign-up page. The new version is even simpler, and it doesn't use a challenge or checkbox. It works invisibly in the background, somehow, to identify bots from humans. Google doesn't go into much detail on how it works, only saying that the system uses "a combination of machine learning and advanced risk analysis that adapts to new and emerging threats." More detailed information on how the system works would probably also help bot-makers crack it, so don't expect details to pop up any time soon.
reCAPTCHA was bought by Google in 2009 and was used to put unsuspecting website users to work for Google. Some CAPTCHA systems create arbitrary problems for users to solve, but older reCAPTCHA challenges actually used problems Google's computers needed to solve but couldn't. Google digitizes millions of books, but sometimes the OCR (optical character recognition) software can't recognize a word, so that word is sent into the reCAPTCHA system for solving by humans. If you've ever solved a reCAPTCHA that looks like a set of numbers, those were from Google's camera-covered Street View cars, which whizz down the streets and identify house numbers. If the OCR software couldn't figure out a house number, that number was made into a CAPTCHA for solving by humans. The grid of pictures that would ask you to "select all the cats" was used to train computer image recognition algorithms.
from Google’s reCAPTCHA turns “invisible,” will separate bots from people without challenges