Captcha or CAPTCHA (completely automated public Turing test to tell computers and humans apart) is a method agains spammers (mostly commentspam) from 2000. The characters (and sometimes backgrounds) are distorted to prevent spamrobots.
In 2008, the 12% of spams were from mailing systems, which shows that it's easier to break captchas.
Captchas can be seen on every pages where it's possible to spam with spambots. The system is fully automatic, only installing requires human.
Main article: Decoding captchasA very commoly used method is mechanical turks, where the software gives the captchas to real people. The operator gives the password, hundreds in an hour. The method is slow, but spammers pay for it.
Other captchas have some errors so it's easy to break them.
Sometimes an OCR-like method breaks the captchas, where the program removes background, separates characters and analysis characters one by one.
This article is translated from Captcha article in Hungarian language.