A CAPTCHA that not only helps protect a site from bots or other scripts, but also helps read books. With a reCAPTCHA, one word is generated by the script and the other is a scanned word that an OCR program didn't understand. The idea is that if a user successfully understands one of the words that it's more than likely that the other word is also correct.
The image shown on this page is an example of a reCAPTCHA showing "were" and "tattoos" as words that need to be entered to gain access. reCAPTCHA has no real way of knowing what word is generated and what word is the scanned word.