what is captcha? Where did you see captcha?

A CAPTCHA is a type of challenge–response test used in computing to determine whether or not the user is human. e.g. of captcha are sum of two digits, selection of charact ers containing symbols, numerals, and alphabets which are distorted for user to identify and write in text field provided.
Selection of pictures that contain some of the images asked for. The pages where sensitive user information may get revealed CAPTCHA can be installed, They prevent hackers from performing attacks where they try to break into an account by guessing every possible password combination, and they can also prevent fraudulent sweepstakes entries or illegitimate contest votes.