CAPTCHA

CAPTCHA is a challenge-response test designed to separate people from computers. It is generally used on the internet to prevent spam on websites and to verify the content of web forms. The CAPTCHA test on www.pc-fix.com is based on simple mathematics, however there are many different CAPTCHA tests in existence; the majority require the user to recognise a distorted piece of text (e.g. reCAPTCHA), while a few are using image recognition to test users (e.g. What's up CAPTCHA [pdf]).

For further information on CAPTCHA, please see the wikipedia article.