What is CAPTCHA ?

As a Website owner you may find you're getting spam in your registration forms, blog and forum comment areas or in your email inbox. And although sometimes it may feel like you spend a lot of your time dealing with these annoyances, there are tools to help minimize or eliminate spambot issues. Many business are adding CAPTCHA codes to their sites in order to help combat this kind of spam.

A CAPTCHA is a program that protects against bots by generating and grading tests that humans can pass but current computer programs cannot. The term CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart," trademarked in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University, who developed the first CAPTCHA.

A common type of CAPTCHA requires the user to type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. This is a simple problem for humans, but a very hard problem for computers that have to use character recognition.

CAPTCHA can be used to monitor and solve many spam-related problems like preventing comment spam in blogs, protecting Web site registration, protecting email addresses from Web scrapers looking to add to spamming lists, ensuring that only humans can vote in online polls, preventing dictionary attacks in password systems as well as stopping search engine bots from crawling unindexed sites.

On the official CAPTCHA Web site (captcha.net) you can download and install a free CAPTCHA code provided by the reCAPTCHA Project that can help with decreasing spam-related issues and enable you to focus on other aspects of your business. ReCAPTCHA also enables you install an audio component to the test, making it user-friendly for the visually-impaired as well.

Some advantages to this code is that is a Web service, which means that all the images are generated and graded by reCAPTCHA's servers and can automatically update whenever a security vulnerability is found. ReCAPTCHA also has an IP address filtering and detection system that flags down IP addreses that solve too many CAPTCHAs in a certain period of time.

Easy-to-install plugins are available for WordPress, MediaWiki, PHP, Perl, Python and other environments. The CAPTCHA code is free for personal as well as commercial Web sites, but if you wish to install a non-branded code, you can purchase it for $160 per year

Resource:
Official CAPTCHA Website
Google ReCAPTCHA Project
http://www.thewhir.com/article-central/Working_With_CAPTCHA