A captcha isn’t a mispronunciation of Chapka, the name for those Russian fur hats. In fact it’s not Russian at all, but an acronym, and a rather tedious one at that: Completely Automated Public Turing test to tell Computers and Humans Apart. All you really need to know is that a captcha is a test designed to make sure that the person posting a message in a forum or a comment on a blog really is an actual flesh-and-blood human and not some malevolent robot. (There’s an example shown below.) In practical terms, the captcha test usually consists of asking you to reproduce a series of letters that pop up in a window. If you use forums or post comments on blogs, or belong to social networking groups or hubs, the chances are you will have done these tests, and now you know the name for them. The technique is used by website editors and bloggers to reduce the risk of spam and avoid having their websites infected by advertising or messages of a political or activist nature.
The captcha is a type of Turing test, designed to detect artificial intelligence. Standard Turing tests are set by a human to be performed by a machine, but a captcha is really a reverse Turing test, meaning that this one is set by a machine to be performed by you, the human. So there you have it.