CAPTCHA is an acronym for "Completely Automated Public Turing Test to Tell Computers and Humans Apart". As the name suggests, it is a test that determines the probability of the solver being a human. As defined on the CAPTCHA home page at the Carnegie Melon University scheool of Computer Science's Web site: CAPTCHA is a program that can generate and grade tests that: - Most humans can pass. - Current computer programs can't pass. Usually Captcha is placed on a web form and it is represented by an image randomly generated. The image rapresnt a dynamic text that the user must copy in a text field. If the verification text is entered incorrectly the application will not allow the user to perform the action. To make more difficult for text recognize application the text is slightly scrambled, rotated and/or appear over patterned background.As a result Captcha protects web sites against unfriendly actions like mass web form spam or brute force password attacks.