What is CAPTCHA here's all you need to know

Captcha full form is Completely Automated Public Turing test to tell Computers and Humans Apart
It is a type of challenge-response test used in computing to determine whether you are a human or not according to wiki this term was coined in 2003 by Luis von Ahn Manuel Blum Nicholas J Hopper and John Langford The most common type of CAPTCHA was first invented in 1997 by two groups working in parallel Mark D Lillibridge Martin Abadi Krishna Bharat and Andrei Z Broder and Eran Reshef Gili Raanan and Eilon Solan.
CAPTCHA is nowadays required in most of the sites it is just because site owner don't want any spammer and they require real user for their website and it is normally require a problem to be solved on the login page so only a human can do it for example read out very blurry distorted numbers in a image solve a maths problem in a image name images and to tell some sign board with identify cars bridge and selecting some pictures and in other hand a theme computer AI is not clever enough to solve these problems.