Did you ever wonder what is the scrambled, distorted text image that websites ask you to read and key in before allowing you to edit any text, make any post in a blog (ex: google blogspot)?
This is called a CAPTCHA.
More on it from wiki...
A CAPTCHA is a type of challenge-response test used in computing to determine whether the user is human. "CAPTCHA" is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart", trademarked by Carnegie Mellon University. A CAPTCHA involves one computer (a server) which asks a user to complete a test. While the computer is able to generate and grade the test, it is not able to solve the test on its own. Because computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. The term CAPTCHA was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University), and John Langford (then of IBM). A common type of CAPTCHA requires that the user type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, a CAPTCHA is sometimes described as a reverse Turing test.
ref : http://en.wikipedia.org/wiki/Captcha
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