What does CAPTCHA mean?

What is a captcha form validation, what is it used for?

Authenticate data is entered by a person, not a computer
“Completely Automated Public Turing test to tell Computers and Human Apart”

Ultimately, a CAPTCHA is used to verify a person and not a computer is entering data into a form on a web page. It is possible to write a program to fill in the fields and submit it for processing. An example would be entering your name and address for a chance to win something. The program, loaded with a list of names and addresses, could be used to submit the entire list of names with the click of a button. A captcha on the form would keep the program from submitting the list automatically as it is hard to create a program to determine what characters or symbols are displayed in the captcha and submit them automatically. A captcha makes it harder for a program to automatically fill in the form by displaying an image consisting of characters, pictures, etc., which a person must then enter into the form for verification. Some captchas will display text and numbers, some display simple math problems, some use symbols.