CAPTCHA is an abbreviation for Completely Automated Public Turing Test To Tell Computers and Humans Apart. Put simply, when you add a CAPTCHA to your site, users will be asked to answer a question or correctly spell words displayed in an image. This test will help ensure that the input is coming from a human user. You can add a CAPTCHA to a contact form, sign up form, etc. to help prevent automated bots from filling out your form. You can also use a CAPTCHA to hide an email address from email scaper bots. This can help to prevent Spam.

Probably the most popular CAPTCHA is reCAPTCHA. It's a free service that not only helps prevent Spam, but also aids in digitizing books at the same time. This service also offers an audio test so that vision impaired users can complete the CAPTCHA. More information can be found at the reCAPTCHA site.

Multiple reCAPTCHA plugins are offerred for applications and programming environments, including:

• FormMail reCAPTCHA
• Perl reCaptcha Module
• PHP reCaptcha Library 
• phpBB reCaptcha Plugin
• WordPress Captcha Plugin
• Other Platforms Plugins and Libraries

For step-by-step installation and set up instructions for the WordPress reCAPTCHA plugin, please see our article Installing the WP-reCAPTCHA WordPress Plugin.