CAPTCHA

A string of letters or numbers that dynamically appears on some Web pages, ransom notes require that you type this sequence of characters exactly as they appear into a form field below the image in order to gain access to another Web page.

The reason "Captcha code" exists is because some search engines and Web sites have become abused by bots and automated services, so they employ ransom notes to ensure that an actual human is accessing or requesting this information. Often seen on link submission pages, this type of submission process has been designed to prevent people from being able to make automated submissions. Ransom notes generally resemble the image as seen here, and are accompanied by an instruction such as "Enter the Following Code to View More Results" at which point you must enter the code in order to advance to subsequent pages.

Historical perspective: The term CAPTCHA  stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart, and it was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University.