UPDATE: A new, better CAPTCHA is available from Google and it's still free. It's called reCAPTCHA.
reCAPTCHA protects your site from spam and abuse. It uses and advanced risk analysis engine to distinguish humans from bots.
With the new API, a significant number of your valid human users will pass the reCAPTCHA challenge without having to solve a CAPTCHA. reCAPTCHA comes in the form of a widget that you can easily add to your blog, forum, registration form, etc.
Read more HERE.
This tutorial is to explain CAPTCHA, an acronym which stands for "Completely Automated Public Turing test to tell Computers and Humans Apart. The purpose of a CAPTCHA is to present a problem that is easy enough for a human to solve, but yet will prevent automated software on a computer from performing whatever tasks the malicious user has in mind.
A CAPTCHA is used to try to prevent automated software from performing actions that degrade a system, either through abuse or resource expenditure. They can be used to help protect against email spamming, and to minimize automated postings to forums and blogs.
CAPTCHA is not perfect, and someone is always working to find a way around the newest versions. They are also not always accessible to the disabled, but attempts are being made to make them so. One of the biggest threats to CAPTCHA is the evil-doers - spammers and hackers - using cheap foreign labor, paying about $.80 to $1.20 for each 1,000 solved CAPTCHA images.
There is a lot of discussion on the internet about the use of CAPTCHA's. Complaints range from being cumbersome, to the new CAPTCHAs that rely on a jumble of mixed up letters, instead of words, making it impossible to guess what that next letter might be.
Many seem to agree that the best approach might be to add some server-wide security to stop generic bots, and watch for suspicious IP's and monitor behavior, such as a large number of requests from one IP, or spam links being sent to many users. Linksky Hosting does employ these methods, but still recommends CAPTCHA's.
The use of CAPTCHA is a personal decision for every site owner to make, but rarely will you hear a complaint about easy to use, added security on your web site.
To obtain a free CAPTCHA script for you web site, click HERE.