CAPTCHA" is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart".
If you posted a comment,or ordered something online you have seen captcha.CAPTCHAs have also found active use in stopping automated posting to blogs or forums.
A method prevent robots from automatically submitting forms on a Web site by requires a person to visually view text in an image and enter its value in the form, which robots cannot do.
The Worst part of CAPTCHA is User's are not able to understand what is written in the image,yet there are some irritating captchas, which results in user gets frustrated and leaves the site. Drop-off rate increases due to bad captchas.
Good captcha should be easy to understand and straight forward.User should be able to view the text in a image when they see.