It is nothing but -Completely Automated Public Turing test to tell Computers and Humans Apart
Web sites are increasingly under spam attack from automated scripts. “CAPTCHAs” help Web sites to distinguish between human and machine users by forming a problem that is easy for humans to solve, but difficult for machines to solve.
CAPTCHA can control many automated spam attacks against Web sites, but without careful planning, it can also cause problems.
A Sample CAPTCHA Image
Because CAPTCHAs rely on perception, users unable to perceive a CAPTCHA (for example, due to a disability or because it is difficult to read) will be unable to perform the task protected by a CAPTCHA.
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