Captcha refers to Completely Automated Public Turing Test to differentiate between computers and humans. A Captcha Security Code is an image of letters that a person must type in to match the Captcha image they see. It is a type of challenge response test used in computing to ensure that the response is not generated by a computer. This code was created to stop automated computer spam robots from filling out forms and harvesting email addresses and then sending out spam emails. Only a human can type out a code he sees correctly, an automated computer programme cannot.