A CAPTCHA (an acronym for "Completely Automated Public Turing test to
tell Computers and Humans Apart") is a
type of challenge-response test to ensure that the response is only generated by humans
and not by a computer.
They
were invented to block spamming machines from posting wherever
they want. In order to keep out spammers, a CAPTCHA has to effectively test
if you are human or machine.
CAPTCPA is mainly used to prevent automated software (bots)
from performing actions on behalf of actual humans.
For example, while signing up for a new email account, you
will come across a CAPTCHA at the end of the sign-up form so as to ensure
that the form is filled out only by a legitimate human and not by any of
the automated software or a computer bot. The main goal of CAPTCHA is to
put forth a test which is simple and straight forward for any human to answer
but for a computer, it is almost impossible to solve.
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