A Captcha is a type of test used in computing wherein the response is not generated by a computer. The process involves a server asking a user to complete a simple test. It is described as a reverse Turning test, since it is administered by a machine to a human.
The image above is a CAPTCHA. Users can interpret it, but computers have a harder time interpreting these letters. Google has acquired reCAPTCHA, a company that creates these CAPTCHAs to help in protecting more than 100,000 websites from spam and fraud. Thus malicious programs cannot scalp tickets or spam on millions of email accounts.
The image above is a CAPTCHA to test whether you are human when you try to register on websites.
Google plans to use the technology adopted by reCAPTCHA both as a security measure with in Google sites and to make its large book-scanning project a smarter one. When these CAPTCHAs come from books and newspapers, computers find it hard to recognize these words because of the degradation in ink and paper over time. Thus reCAPTCHA`s unique technology converts these scanned images as plain text which is known as Optical Character Recognition (OCR). Google Books and Google News Archive Search websites will also use this technology. We can also easily search for information, render information in mobile devices and use the information for visually impaired persons when we have the text version of the document.