A CAPTCHA is a type of challenge–response test, a program or system intended to distinguish human from machine input, typically to thwart spam and automated extraction of data from websites.
The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford. The most common type of CAPTCHA was first invented in 1997 by two groups working in parallel.
CAPTCHA technology authenticates that a real person is accessing the web content and blocks spammers and bots that try to automatically harvest email addresses or try to automatically sign up for access to websites, blogs or forums. CAPTCHA blocks automated systems, that can’t read the distorted letters in the graphic.
reCAPTCHA, a form of CAPTCHA, is a free Google service.
hCaptcha is another form of CAPTCHA and an alternative to reCAPTCHA.
On a WordPress site, they can be used to protect forms and comments