Technology speaks in volume also. By now you may have saw variety of captcha's at various form. CAPTCHA is used to prevent spammer auto fill forms using software.
reCAPTCHA acquisition is not only for captcha but it has another technology under its arm - Reading scanned pages from book. reCAPTCHA has a technology which is good in reading scanned copy from book.
reCAPTCHA images looks like this:
More here at Google