The Captcha plugins are used to prevent spam submissions on your forms such as registration, contact and login. You basic installation of Joomla includes one Captcha plugin which leverages the ReCaptcha® service but you may install other plugins connecting to different Captcha systems.
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Note: ReCaptcha is a the trademark of Google Inc. and is an independent product not associated with or endorsed by the Joomla Project. You will need to register and agree to the Terms of Service at Recaptcha.net to use this plugin. Complete instructions are available if you edit the ReCaptcha plugin in the Plugin Manager.