Captcha is known for being a spam prevention effective solution in regards to all forms of spam that can be done by bots. The reason for that lies in the fact that Captcha is a test that validates web users as human beings, just as the name implies: CAPTCHA = Completely Automated Public Turing test [...]

Audio Captchas are an alternative for people who suffer from sight impairment and thus are unable to solve regular textual or image Captchas

Captcha is known for its efficiency in ensuring proper use of websites and online application by validating that the user is human and not a bot or some other automatic machine.
This article lists some of the security applications / uses of CAPTCHA:
Comments spam prevention
Comments spam is used in blogs and forums mostly for raising search [...]

Captcha protects your website from unwanted users such as automated machines, bots and spammers. Once you have decided to use Captcha for your site, there are some important criterions you must take into consideration:
Accessibility
Captcha must be accessible for every user – for the normal and the blinds.
That means that the captcha image shouldn’t be too [...]