CAPTCHA

The word CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart".
CAPTCHA is a type of challenge-response test used in computing to determine whether a user is being imitated by a computer. The process 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. A common type of CAPTCHA requires that the user type the letters displayed in a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen.

A CAPTCHA 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.

Currently, reCAPTCHA is recommended as the official CAPTCHA implementation by the original CAPTCHA creators.

Applications:
CAPTCHAs 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. Although CAPTCHAs are most often deployed as a response to encroachment by commercial interests, the notion that they exist to stop only spammers is mistaken. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam, such as the webmail services of Gmail (there is also a claim that the Gmail CAPTCHA has been compromised), Hotmail, and Yahoo!. CAPTCHAs have also found active use in stopping automated posting to blogs, forums and wikis, whether as a result of commercial promotion, or harassment and vandalism. CAPTCHAs also serve an important function in rate limiting, as automated usage of a service might be desirable until such usage is done in excess, and to the detriment of human users. In such a case, a CAPTCHA can enforce automated usage policies as set by the administrator when certain usage metrics exceed a given threshold. The article rating systems used by many news web sites are another example of an online facility vulnerable to manipulation by automated software.