If you allow users to publish content on your website, from leaving comments to creating user profiles, you’ll likely see spammers attempt to take advantage of these mechanisms to generate traffic to their own sites. One way to determine which of your visitors are human is by using a CAPTCHA , which stands for "completely automated public Turing test to tell computers and humans apart." You can easily take advantage of this technology on your own site by using reCAPTCHA, a free service owned by Google. By using reCAPTCHA, you're not only protecting your site from spammers; you're helping to digitize the world's books. Luis Von Ahn, one reCAPTCHA's co-founders, gives more details about how the service works in the video below:
f you’d like to implement reCAPTCHA for free on your own site, you can sign up here. Plugins are available for easy installation on popular applications and programming environments such as WordPress and PHP.