What is a CAPTCHA?
The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. At the time, they developed the first CAPTCHA to be used by Yahoo.
A CAPTCHA is a used as a challenge to automated entries and robots (bots) to ensure that human is entering the information and the data is not being entered by a computer.
CAPTCHAS are used to prevent unauthorized logins, signups, and also help prevent spam. Computers and bots are becoming so sophisticated, some can now defeat single CAPTCHAS. So Web sites are now using double-CAPTCHAS, like the one below: