CAPTCHA is an acronym standing for Completely Automated Public Turing Test To Tell Computers and Humans Apart.
Captchas are used to prevent bots to register fake accounts at scale for different free online services - webmail accounts, social media accounts, forum & comment profiles, etc.
A traditional captcha requires people to type in a set of distorted letters and numbers appearing in an image and difficult for a bot to decipher
From a marketing standpoint, a captcha is often a strong deterrent or obstacle for real users. It must be used with caution for preventing a loss of a registrations. For instance, Twitter asks only people trying to create multiple accounts from a single I.P. address to answer a “captcha.”
To prevent frustated users, text-based captchas are increasingly replaced by image-based or game-based human verification technologies.
Visual mobile CAPTCHAs are in particular used on smartphones.
Many captcha breaking techniques have been developed and it leads captcha images to be more and more difficult to decipher for humans.
In some rare situations captchas are used as advertising vehicles: see captcha advertising.