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CAPTCHA is the human validation tes (usually the blurry squiglly letters that need to be deciphered) t used by many sites to prevent spam.
reCAPTCHA is a reversed CAPTCHA - the same test, used not only to prevent spam but to help in the book digitazion project. In other words, the reCAPTCHA tests are not meaningless combination of words, but excerpts from books that undergo digitation, while CAPTCHA uses several human validation methods including math or general knowledge questions, visual puzzles and even chess puzzles.
Google purchased reCAPTCHA several years ago, and not it is also used to collect street view data.
reCaptcha is hosted by Google, and one of the more interesting things about it is that it is used to digitize text of old newspapers and books. That’s why there are two “sections” of a reCaptcha instead of the single series of characters for CAPTCHA - one is known text, the other is not. If you get the known one correct, it assumes you got the second one. Then the next time it offers up that same “unknown” text, it is considered possibly known. A few more times with the same result for the “unknown” text, and it becomes “known” and the text it originated from can be correctly digitized. Clever, eh? Also, because of frequent updates, I would expect reCAPTCHA to be slightly better at preventing bots from solving them.