You've seen this picture before, haven't you? Websites and popular platforms use this "Are you a human" detection method to avoid bots from spamming or brute-force passwords. It is a free and quite reliable way and anyone can use it, but how free is it really?
Well, in terms of payment, it is indeed free, but in terms of labor you actually participate as a volunteer to help Google digitize books.
OCR stands for Optical Character Recognition. These are specially designed software engines which take scanned images and try to detect any text inside and then convert it into digital information. But they do not work that well. Letters may not be clear, the ink may faint, the page may become yellowish...
reCAPTCHA was a company which was bought by Google. The goal is to put real human volunteers do the recognition. Which explains why there are 2 words to decipher. One word is the actual "Are you a human" engine. The other word may come from an unrecognized scanned portion of a document or book. This word will appear over an over again to millions of humans (hey, this is Google remember?). The results will then be compared and voila! The text is recognized, partly thanks to you.
Watch this very interesting video which gives more details about reCAPTCHA