A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) 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 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.
CAPTCHA’s are used to prevent automated software from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure.
Additional information and resources are available at: http://www.captcha.net