CAPTCHA
CAPTCHA
means Completely Automated Public Turing test to tell Computers and Humans Apart.
A server uses a CAPTCHA to verify that a client is a person and not a machine. A CAPTCHA is often visual, like the one shown below, a distorted image of some characters that defeats optical character recognition (OCR) and that the user has to type in. A CAPTCHA can also be a simple question, such as a simple math problem like "3 + 5", where the user has to type in the answer.
I mention CAPTCHAs in Web 2.0: APIs Instead of GUIs. Web sites use CAPTCHAs to prevent bots from using the Web site automatically, so that only humans can use the site. This helps prevent automated commenting such as spam, prevent polling for prices or status, prevent automated account creation, etc.
See also:
Some CAPTCHA implementations:
Luis von ahn
of Carnegie Mellon University has interesting research on Human Computation, which basically utilizes computational abilities of humans to perform tasks that cannot be easily accomplished by computers. One of the best examples of it is labeling images. Google Image search uses this to label images.