CAPTCHA or Captcha which is the short form of “Computer Automated Public Turing test to tell Computers and Humans Apart” is used to check whether the response is from human and not by a computer.This is simply a word verification test which one willl experience at the end of any sign-up form..now a days we can see this even while downloading any file..User will be prompted to enter a word being displayed…if its rite further permissions will b granted.CAPTCPA is mainly used to prevent automated software (bots) from performing actions on behalf of actual humans.For many the CAPTCHA may seem to be silly and annoying, but in fact it has the ability to protect systems from malicious attacks where people try to game the system. Attackers can make use of automated softwares to generate a huge quantity of requests thereby causing a high load on the target server which would degrade the quality of service of a given system, whether due to abuse or resource expenditure. This can affect millions of legitimate users and their requests. CAPTCHAs can be deployed to protect systems that are vulnerable to email spam, such as the services from Gmail, Yahoo and Hotmail.CAPTCHAs are mainly used by websites that offer services like online polls and registration forms. For example, Web-based email services like Gmail, Yahoo and Hotmail offer free email accounts for their users. However upon each sign-up process, CAPTCHAs are used to prevent spammers from using a bot to generate hundreds of spam mail accounts…This Turing test can easily be solved by any human, but almost impossible for a computer.