CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart". It is a technology that attempts to sort out the people from the programs.
This is necessary, because there are many malicious programmers out there that write programs to automatically submit forms, including logins, to circumvent security measures, gather logins, deface websites, steal data, and much more. What CAPTCHA does is place an image on the web page, and asks the visitor to type the letters and numbers into a textbox to verify that they are human. The thought is that a computer program would not be able to decipher the characters from the rest of the image, thereby identifying the visitor as human.
On a DotNetNuke website, the CAPTCHA challenge might look something like this:
In order to enable CAPTCHA to protect your login form, you need to first login as an Admin for the website. At this time, CAPTCHA must be enabled individually for each portal on a DNN installation. Once logged in, choose "Authentication" from the Admin menu.
In the Authentication module, you should immediate see the option to check to enable CAPTCHA for your website. Simply check the checkbox, click the Update link. Your work is done. Log off and test it out!
DNN, DotNetNuke, DNN Blog, How To, Administration, 4.09.01, 5.00.00