CAPTCHA is first and foremost an acronym (the computer industry loves ’em) for “Completely Automated Public Turing test to tell Computers and Humans Apart”.
Now many people reading this (including yours truly when I was writing this) may still be asking what does that mean? Well the “Turing test” was created by renowned Mathematician Alan Turing in 1950. Many believe Turing’s pioneering work in early Computational Philosophy and theory created the foundation of what people now consider the field of Computer Science. In short, he created a basic test to determine elements of artificial intelligence and to determine whether or not a human was reading something or if a computer was reading something. You can see Benedict Cumberbatch as Alan Turing in the 2014 feature film “The Imitation Game“.
The brief answer to “What is CAPTCHA” is that it is a system of using images of words and/or letters that are distorted (in some way or another) to make them difficult for computers to read but possible for humans to read. When a human enters the characters displayed by a CAPTCHA system, the system grades the entered characters and either allows the process to continue or blocks it and requests another test from the user. This automated system greatly reduces computerized systems from flooding things like mail and web servers with artificially created entries in things like web forms.
You can see an example of a CAPTCHA system in use on our Contact page. We use it to try to prevent automated systems from flooding our mailboxes with fake requests.