What is "captcha"

CAPTCHA is an acronym for the expression "Completely Automated Public Turing test to Tell Computers and Humans Apart" (a fully automated public Turing test for differentiation between computers and humans): a cognitive challenge test, used as an anti-human toolspam, pioneered at Carnegie-Mellon University.

A usual CAPTCHA involves a computer (a server) that asks a user to complete a test. As computers are unable to solve CAPTCHA, every user incorporating a correct solution is presumably human.

Source: Wikipedia