CAPTCHA stands for “Completely Automated Public Turing test to tell Computers and Humans Apart”. It was debut widely on the internet about 2 years ago to fight the problem of increased spam registration. CAPTCHA was essentially helpful for e-mail services, chat rooms, forums and blogs that needed to screen out spam registration.
Spammers in the early days used automated software to register accounts at forums and blogs sites, then use these accounts to spread their links and spam. However, the introduction of CAPTCHA severly limited their abilities to automate registration. CAPTCHA displays an image with letters or numbers and request from the visitor to identify the phrase in that box. At first, this had represented a challenge to spammers as their software was not designed to read images. Nevertheless, few years after the wide spread of CAPTCHA at popular services such as GMail and HotMail, spammers have finally found a very successful method to break CAPTCHA.
This is an interesting article that discuss why CAPTCHA is no longer as relevant as it was few years ago. CAPTCHA though is still a great way to reduce spam on your own Blog/Forum, however, if an attacker is obsessed with compromising your system, there is technically nothing stopping them: