Your Captcha plugins are used to prevent spam submissions of your forms such as registration, contact, login, forgotten password, forgotten Username
Yoru basic installation of Joomla!® includes one Captcha plugin which leverages the ReCaptcha® service but you may install other plugins connecting to different Captcha systems if you desire.
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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.
In order to use ReCaptcha® you will likely need to create an account with Google and obtain two domain specific "keys" - one Site key, and one Private (secret) Key and enter these keys in their respective place within your ReCaptcha plugin in order for captcha-Recaptcha to work.