CAPTCHA is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart.” It’s trademarked by Carnegie Mellon University.
Technically, a CAPTCHA is any problem or test displayed by a computer that, in theory, is beyond the capabilities of current computational technology to figure out in a reasonable amount of time, and yet something that is relatively easy for an actual human being to understand.
The most common forms of CAPTCHA are visually distorted letters, where the user is instructed to type in the letters displayed. This type of analysis is currently practically impossible for computers to perform.
CAPTCHAs are most frequently used to prevent automated programs from creating new accounts at various services, or to prevent automated programs from sending spam.