Captcha Explained

A CAPTCHA is a type of challenge-response test used in computing to determine whether a user is human or not. It is an technique that prevents spammers and scammers from leaving fake comments on blogs, forums or social media or other sites.
 

A captcha often consits of oddly shaped letters and numbers, some also have distortions in the background, playing with colors and blurs. that appear in an image and typing them out. "CAPTCHA" is a contrived 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).