from Wikipedia ( http://en.wikipedia.org/wiki/Captcha ):
A captcha (an acronym for "completely automated public Turing test to tell computers and humans apart") is a type of challenge-response test used in computing to determine whether or not the user is human. The term was coined in 2000 by Luis von Ahn, Manuel Blum, and Nicholas J. Hopper of Carnegie Mellon University, and John Langford of IBM. A common type of captcha requires that the user type the letters of a distorted and/or obscured word that appears on the screen.
A captcha might take a word such as 'year' and obscure it by adding a circular ripple effect, some distortion, and a coloured border...
They are used to prevent bots from using various types of computing services. Applications include preventing bots from taking part in online polls, registering for free email accounts (which may then be used to send spam), and, more recently, preventing bot-generated spam by requiring that the (unrecognized) sender successfully pass a captcha test before the email message is delivered.
I wrote a simple CAPTCHA script, using LaTeX, Perl Crypt module and Image-Magick convert and mogrify functions, for protecting Noti-AMCA users email addresses from spambots. If someone is interested, please write me (mario.storti@...).
-- MarioStorti - 04 Mar 2004