How CAPTCHAs Usually Work?

[guolo]

Could anyone explain how captchas work?
Is there any particular format of or we can use any of captcha to our website?

 

[johnpitter]

CAPTCHA is a Abbreviation of Completely Automated Public Turing test to tell Computers and Humans Apart. A CAPTCHA image shows a random string which the user has to type to submit a form.

 

[domaindiving]

captcha is one tool that protects a certain site from spam bots that could identify one's profile especially on forum sites. You can search good tools of it online if you want to apply them to your site.