CAPTCHA stands for “Completely Automated Public 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 want to prevent spam submissions.

CAPTCHAs can take a variety of forms, including asking logic questions,
presenting skewed fonts, and presenting multiple images and asking how they
relate. The Zend\Captcha component aims to provide a variety of back ends
that may be utilized either standalone or in conjunction with the Zend\Form
component.