A CAPTCHA is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. 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 assumed to be 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 the user to type letters or digits from a distorted image that appears on the screen.
The term "CAPTCHA" was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University). It is an acronym based on the word "capture" and standing for "Completely Automated Public Turing test to tell Computers andHumans Apart". Carnegie Mellon University attempted to trademark the term,[2] but the trademark application was abandoned on 21 April 2008.[3]
Characteristics: A CAPTCHA is a means of automatically generating challenges which intends to:
Provide a problem easy enough for all humans to solve.
Prevent standard automated software from filling out a form
For more information: please go to the below link:
http://en.wikipedia.org/wiki/CAPTCHA
