…more basic than authentication itself: the very basic thing about identity is, in my opinion, proving that you are a human. This is especially useful in order to prevent machines from filling registration forms (and posting blog comments etc.)

A CAPTCHA ("Completely Automated Public Turing test to tell Computers and Humans Apart") is a type of   test used to ensure that the answer is generated by a human being.

I've first read about the idea on Taylor Hayward's blog, but it looks like someone else (have a look at their registration form) was working on the same idea and made it real, although I preferred the animal-styled one…