A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't:CAPTCHA example
The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. Get a Free CAPTCHA For Your Site
A free, secure and accessible CAPTCHA implementation is available from the reCAPTCHA project. Easy to install plugins and controls are available for WordPress, MediaWiki, PHP, ASP.NET, Perl, Python, Java, and many other environments. reCAPTCHA also comes with an audio test to ensure that blind users can freely navigate your site. reCAPTCHA is our officially recommended CAPTCHA implementation. Test Drive a CAPTCHA
reCAPTCHA. Stop spam and help digitize books at the same time! The words shown come directly from old books that are being digitized. SQUIGL-PIX. Our newest CAPTCHA! ESP-PIX. A CAPTCHA script that's close to our hearts. Instead of typing letters, you authenticate yourself as a human by recognizing what object is common in a set of images. This was the first example of a CAPTCHA based on image recognition.