reCAPTCHA is an anti-spam method originating from Carnegie Mellon University, then acquired by Google which uses CAPTCHAs in a genius way. Instead of randomly generating useless characters which users grow tired of continuosly typing in, risking the possibility that spammers will eventually write sophisticated spam bots which use OCR libraries to read the characters, reCAPTCHA uses a different approach.
The world is in the process of digitizing books by use of automated machines which employ the use of Optical Character Recognition software. Sometimes the certain words cannot be read by the software. reCAPTCHA uses a combination of these words, further distorts them, and then constructs a CAPTCHA image. After a certain percentage of users solve the 'unknown' word the same way, it is assumed that it is the correct spelling of the word. This helps digitize books, giving users a reason to solve reCAPTCHA forms. Because the industry level scanners and OCR software which are used to digitize the books can't read the words with which the CAPTCHAs are constructed, it is safe to assume that in-house spam-bot OCR techniques will not be able to bypass the resulting CAPTCHA, which is a further distortion of the unreadable word.
reCAPTCHA is probably the most popular and widely accepted CAPTCHA systems by both end-users and site-owners. It is used by such sites prominent sites as Facebook, Twitter, to the Average Joe's little blog out there on the corner of the Internet.
It is accessible by everyone. If the user has trouble reading the CAPTCHA challenge, he or she has the option of requesting a new one. If this does not help, there is also an audio challenge which users may use.
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source:http://trafficneed.blogspot.com/2012/06/13578665064015.html