CAPTCHA is an abbreviated form of a "Completely Automated Public Turing test to tell Computers and Humans Apart".  Since such an awkward acronym is hard to remember, it may be better to think of it as a "Completely Automated Process for Telling Computers and Humans Apart".  Whichever way you remember it, CAPTCHAs, pronounced like "capture" with a New York accent, are mechanisms used to distinguish human beings from computer programs.  The importance of this is clear when considering online forms that allow one to, let's say, email a friend to let them know about an interesting page on your web site.  Without a way to distinguish human beings from computer programs the online form could be used to send out spam emails from your web server.  In order to prevent this, CAPTCHAs require people to perform an action that is very easy for a human being but very difficult for a computer program.  Most often CAPTCHAs take the form of a mottled graphic containing a set of characters that need to be typed into a corresponding box.  However, there are other forms of CAPTCHAs, such as an audio clip spelling out characters which need to be typed into a text box or even a video clip or animation.  But in all cases, CAPTCHAs must have a means for a human to discern a correct value and enter the correct value to verify that they are indeed a human being.

last updated 2007.09.27


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