Not case sensitive - All one word or with a space between.

Why CAPTCHA?

To Reduce the amount of 'Form Spam' from automated submissions.

Why reCAPTCHA?

By entering the words in the box, you are also helping to digitize texts that were written before the computer age. The words that you see were taken directly from old texts that are being scanned and stored in digital format in order to preserve them and make them more accessible to the world. Since some of the words in these texts are difficult for computers to process, we are using the results of your efforts to help decipher them.

WIKI:

A CAPTCHA or Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Thus, it is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human, in contrast to the standard Turing test that is typically administered by a human and targeted to a machine. A common type of CAPTCHA requires that the user type letters or digits from a distorted image that appears on the screen.