CAPTCHA (an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”) is a type of challenge-response test used in computing to determine whether or not the user is human.
CAPTCHA is a must for fighting spammers who seek to use the forms on your website to deliver their message, viruses and other unwanted deliveries.
In order to add CAPTCHA to Contact Form 7, you need to register the website with ReCaptcha, which is now owned and operated by Google.
Once you register and received your Site Key and Secret Key, go to the Contact plugin (not form), select Integration from the drop down, and the add the keys. Then go to your form and insert the tag [recaptcha] to the place on the form where you want the CAPTCHA to appear.