A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security system that distinguishes human users from automated bots or scripts. It is used on websites to restrict access to certain functions for human users. CAPTCHAs consist of hard-to-read letters, numbers or images that the user must enter or identify. They represent a hurdle for bots and protect website functions such as contact forms from misuse.