CAPTCHA stands for "Completely Automated Turing test to tell Computers and Humans Apart;" it is used as a challenge-response to ensure that the individual submitting information is a human and not an automated process. Typically, a captcha is used with form submissions where authenticated users are not necessary, but you desire to prevent spam submissions.

Captchas can take variety of forms, including asking logic questions, presenting skewed fonts, and presenting images and asking how they relate. Zend_Captcha aims to provide a variety of backends that may be utilized either standalone or in conjunction with Zend_Form.