Sabe aqueles números e letras que pedem para você digitar antes de alguma operação no computador para evitar entradas automáticas?
Isso se chama CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). A category of technologies used to ensure that a human is making an online transaction rather than a computer. Developed at Carnegie Mellon University, random words or letters are displayed in a camouflaged and distorted fashion so that they can be deciphered by people, but not by software. Users are asked to type in the text they see to verify they are human.
Source: http://www.answers.com/topic/captcha-1#ixzz1RSexLUe2