Your Captcha plugins are used to help prevent spam submissions of your forms such as registration, contact, login, forgotten password, forgotten Username.
Captcha is an acronym for: completely automated public Turing test to tell computers and humans apart.
Captcha is a test used in computing to determine whether or not the user is human.
Captcha software can help because it can block robot software from falsely submitting forms - although, like anything, some unscrupulous people endeavour to create programs that can cheat (or beat) captcha with varying degrees of success. Perhaps a bit like a lock picker.
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.
Default state on:
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.