CAPTCHA is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart.” A CAPTCHA is a visual device that ensures that a human being, rather than a computer, is interacting with the site. CAPTCHA can be used for both the Admin and customers.
Users can reload the displayed CAPTCHA by clicking the Reload icon in the upper right corner of the image. The number of reloads is not limited.
The Magento CAPTCHA was implemented using a Zend Framework module, and is fully configurable.