You know CAPTCHAs? Those annoying, weird, funky, little letters that you have to enter everywhere nowadays to leave a comment or sign up for an account. They're just there to prevent spamming and bots signing up, by verifying that you're a real human being.
Now, wouldn't it be nice if this annoyance would at least provide some benefit? That's exactly what
reCAPTCHA is about. The difference to a normal CAPTCHA: it presents you two words instead of just one weird five-letter combination. The one word it already knows about and again, it is there to verify you as a real user. The other one, though, is a scanned picture from an old book which is currently being digitized, and the CAPTCHA system doesn't know what the word actually reads. But it's really easy for you to identify, so you help out by also entering the second word, which gets stored and helps recovering and digitizing old books. Awesome.
