CAPTCHA is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”. These are often used by sites to prevents bots or web crawlers from filling up forms often with the intention of posting unwanted text, or to break passwords by doing brute-force matching. The general idea is the image or pattern is something that can be discerned by a human but is very difficult to understand for a machine. But the images are often so warped that even humans will have a hard time understanding them. I know I've encountered several of those myself.
A company called NuCAPTCHA is introducing a video-based method that is supposed to make it even more difficult for machines to read the message, but is very clear to a human. It uses Flash video, so forget the Apple products (iPhone, iPad, etc.) which do not support Flash. There's a whole bunch of API's available at the site which can be used to integrate across a broad range of platforms include a Wordpress plug-in.
Hmmm... maybe I should consider embedding this into Dragonpay for online payment authentication.