Definition: CAPTCHA is a trademarked term from Carnegie Mellon University and stands for "Completely Automated Public Turing test to tell Computers and Humans Apart."
CAPTCHAs are those little boxes of distorted text that you need to type out in order to register for many Internet services.
Basically, CAPTCHAs attempt to tell if someone on a Web site is human or a computer. Spammers have been known to send automated comments to blogs and guest-books in an attempt to raise a site's PageRank.
Google is experimenting with audio CAPTCHAs for users with visual impairments.
