Google recaptcha is a tool which is free of cost available on google site. http://www.google.com/recaptcha
First you should know what is captcha and why we used it in forms of websites.
Captcha is a picture of broken words mostly shown in the box and it’s placed in the form .
Due to large spam posting on websites, we used captcha to minimize the spams.
Captcha reduced spamming on websites, mostly on blogs, where commenting played a wide role of their popularity.
If you are using CMS wordpress, drupal ,joomla) for your website, then must install a plugin which give you inserting captcha in your forms.