The primary function of a CAPTCHA system is to distinguish between human users and automated bots. This is achieved by presenting challenges that are easy for humans to solve but difficult for machines, like identifying distorted text, selecting images, or answering simple questions. CAPTCHAs enhance security by preventing bots from engaging in malicious activities, such as spam submissions or unauthorized access to online accounts. They are commonly used in login forms, online registrations, and various web services to protect sensitive information from automated threats.