I’m sure you have encountered captchas in the internet, you are just not aware with its name or how does it look like. Commonly these are images that are computer generated that display a combination of numbers or letters or mixed. These are used on blogs, or forum registrations in which we have to type to continue or to complete the registration process.
CAPTCHA stands for Completely Automated Public Turing-test to tell Computers and Humans Apart. It will recognize if you are just using a bot or automated software to register or comment with sites that uses captchas because you can’t solve it by any means. If you are a real human being solving captchas is very easy as you just need to copy what is shown or just answer what is being asked. CAPTCHAs test prevent about 90% of hackers attacks.