Captcha is a plugin that you can include in your forms to make sure that the responses you receiving are generated by a human being and not an Internet bot.
Captcha displays distorted letters, numbers and other digits in a field and asks human beings to reproduce those characters using their keyboard. Captcha is generally used to prevent Internet bots from accessing a site and using a form to register, create accounts, hack sites or send SPAM.
By using Captcha, you can lessen your risk of this occurring to you. This article explains how to enable Captcha for your Joomla site.
Log into your joomla administration section
After logging into the back end of your Joomla site, you'll first want to go to the Global Configuration area to select a default captcha service.
Set Default Captcha
Under the site tab, find the Default Captcha option and select "Captcha-ReCaptcha" from the drop down menu. Be sure to save your work. There are other captcha extensions and services that you can use, but ReCaptca is widely considered to be on of the best free captcha services available, and it is already a core Joomla feature so there is no need for additional 3rd party software.
Next, go to Extensions > Plug-in Manager to enable and configure the Captcha-ReCaptcha plug-in.
Plugin List View
Scroll down and locate the Captcha-ReCaptcha plug-in and open it.
Enable The Plugin
Under the "Details" tab, make sure that the plug-in is enabled. Then, click on the "Basic Options" tab.
The Basic Options Tab
You'll see fields for your Public and Private Keys. These are the two keys we will need from Googles reCAPTCHA service for our captcha to work.
Open a new tab in your browser and Visit the Google ReCaptcha site to generate Public & Private Keys.
As you follow the steps, you'll see a field to enter your domain name. As you do so, it's a good idea to create a global key - to enable the keys to work on all pages associated with the domain name that you enter. When you're done, click on the "Create Key" button.
Now you will be given a set of two keys.
Front end View
Now you have pasted the keys in their respective fields click the save button.
Your reCAPTCHA plugin should now be up and running.
Front-End Form View
Now go to your website front end, if you have any forms created already you will see the Captcha enabled for that form.
If you don't have any forms on your website yet then don't worry, once you create a form Joomla will automatically install the Captcha on that form for you.
Return to the Captcha-ReCaptcha plug-in in the Plug-in Manager and open up the "Basic Options" tab again. There are a few themes to pick from style your captcha form.
Refresh The View
Now go to your front end once more and refresh the page.
You will see the different theme.