Captcha

Introduction
A Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer.

On the Wikipedia you can find more information about this topic.

Web sites that accept postings from the public, like this wiki, are often abused by spammers who use automated tools to post their links to many sites. While these spam links can be removed, they are a significant nuisance.

If you're an anonymous user (not loggen in), the wiki will show you an image of colored or distorted text and ask you to type the words shown when you try to edit a page or register for an account on this site. This is called a captcha test. Since it is a task that's hard to automate, it will allow most real humans to make their posts while stopping most spammers and other robotic attackers.

An example of use of Captcha in action is here

Currently, Captcha creators recommend use of reCAPTCHA as the official implementation.