what is captcha ??

thanks satyapriya nayak 

Captcha is a code ,so that the bots do not fill the forms by any unauthorized access or information which needs to be done by an human .For more information you could check the following site for Captcha

thanks friends for replying 

but how can a Captcha differentiate human and robot ? because if computer generate the captcha then computer can also fill the captcha..  

hello sanjay!

in most common and simple words, captcha is-
'a computer program that is used in webs in order to check whether, a interacting element to the web is a human being or any robotic element.'

benefits of captcha are:
- Differentiating between human and robotic elements
- Security concerns
- Authentication

for more info u can, visit official website of CAPTCHA.
:)

http://en.wikipedia.org/wiki/CAPTCHA
http://www.captcha.net/