Captcha are automated systems that their cause is to prevent a computer program to enter a website;
This is useful for many reasons and one of them is to prevent programs to create fake accounts automatically

How does it work? A picture is displayed and the user has to enter the text written on the picture to a input box and submit it. If the text is right the user gets access to the following page.

Some examples :

This procedure is very easy to humans and at the same time very difficult for programs.

Or it isn't ?

This page demonstrates that the captcha test can be easily bypassed by a computer program.
Find a captcha from anywhere, or make one, as complicated as you want, and submit it. In some seconds you will have the answer.

Pay to use our Captcha Cracker

The procedure finds the right text with a high success rate.
The code that is used is ~15 lines.
Notice that this method is used by all major internet companies such as Yahoo, Google, Rapidshare etc, and at the moment there is no better way to tell people from programs.