Captcha
Also known as: Completely Automated Public Turing test to tell Computers and Humans Apart
CAPTCHA is an abbreviation for "Completely Automated Public Turing test to tell Computers and Humans Apart." The term refers to techniques to tell computers and humans apart, for example, in Web site forms. For example, captchas are widely used in reaction functions in weblogs and in contact forms to prevent spam.
Captchas can also be used to prevent bots from accessing certain functionality, such as free webmail services (which can be used to send spam ), online calculation tools and influencing online polls.
The best-known manifestation of captcha is the retyping of a word or series of characters. This text is wrapped in a picture, with the letters often distorted. Reading and retyping the text is usually no problem for humans but a lot more difficult for computers.
Captchas can indeed be cracked. Spammers today make their bots so intelligent that they can indeed read the text in images. A watertight captcha is therefore a delicate balance against complexity against bots and usability for the visitor. In general, captchas are not conducive to the accessibility and usability of a Web site.
