What Is "Captcha" and How it works ?

captcha, you are seeing this word in network mostly ,

Firstly we are going to know abbreviation 

Captcha:Completely Automated Public Turing test to tell Computers and Humans Apart.this is the abbreviation of captcha .

CAPTCHAs are tools you can use to differentiate between real users and automated users, such as bots. 

Yeah and that's the reason of captha.