From wikipediaA CAPTCHA ( /ˈkæp.tʃə/) is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. The process usually involves a computer asking a user to complete a simple test which the computer is able to grade. These tests are designed to be easy for a computer to generate, but difficult for a computer to solve, so that if a correct solution is received, it can be presumed to have been entered by a human. A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen, and such tests are commonly used to prevent unwanted internet bots from accessing websites.
DotNetNuke introduced it's own captcha control with the release of 3.0. Over time it's been integrated into a number of places, typically via a setting that makes it optional. It can be required for login, registration and associating logins with existing authentication systems.In addition many modules (both core and 3rd party) have also integrated it.