A CAPTCHA is a small visual puzzle used to identify if the user of a website is human or a computer program. CAPTCHAs work by displaying distorted text that the user must then type in. Since no computer program can read distorted text as well as humans can, the web site or program is effectively protected from automated programs ("bots") that are frequently used for sending spam and other malicious uses.
To archive human knowledge and to make information more accessible to the world, multiple projects are currently digitizing physical books that were written before the computer age. The book pages are being photographically scanned, and then transformed into text using "Optical Character Recognition" (OCR). The problem is that OCR is not perfect. reCAPTCHA improves the process by sending words that cannot be read by OCR to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA.