"Perhaps the most loathed of all Internet security measures, a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) attempts to weed out bots from legitimate users by presenting a test that is easy for a human, but difficult or impossible for a computer."

"CAPTCHAs are a nice idea in theory. They’re meant to keep spam comments at bay and prevent bots from harvesting email addresses. They’re also very commonplace; according to Luis von Ahn, one of the creators of CAPTCHA, about 200 million CAPTCHAs are completed every day. But they present some serious user experience problems. We tested the CAPTCHA experience with our panel, and here’s what we learned:"

http://tinyurl.com/mhj8hro