This post no longer reflects the state of the art. See here .
CAPTCHAs – “Completely Automated Public Turing test to tell Computers and Humans Apart”s – are known as the foremost means to stop registrations by SPAM programs, so-called “Bots”. In phpBB, a visual confirmation CAPTCHA is used.
The key here is the “Completely Automated” part, meaning that the software – phpBB – creates the question and the correct answer without interaction by a user. This has the drawback that computers are usually able to find the answer as well, given time to adjust. This is an active field in research. In the end it is an arms race. A new CAPTCHA will usually buy a few months of peace, before the major Bot vendors adjust their products.
This article is about presenting some reasons behind our CAPTCHAs; it is not intended to be a case for or against CAPTCHAs in general or particular. It is not about other means to combat SPAM, but only about CAPTCHAs.