How does reCaptcha help digitize books ..

	Type the two words:

The words above come from scanned books.  By typing them, you help to digitize old texts.

Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the 

one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct. 

More: http://recaptcha.net/learnmore.html