When having a registration page on your Joomla site it is highly recommended that you put a CAPTCHA to further secure it and ensure that only real users register. Otherwise, your Joomla might be targeted by spam bots, which can cause you various website and server issues.
To enable Captcha, you need to log into your Joomla Administrator panel -> Extensions -> Plug-in Manager and select the Captcha – ReCaptcha.
You will see there that it requires a Global or a Private key.
To get a global or a private key for your domain, go to http://www.google.com/recaptcha.
Once you get your key, return to the Captcha – ReCaptcha management page, enter it and hit Save.