CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security measure used on websites to distinguish human users from automated bots. Common CAPTCHAs ask users to identify objects in images, click checkboxes, or solve simple puzzles—tasks easy for humans but difficult for computers. CAPTCHAs prevent automated abuse like spam submissions, fake account creation, or overwhelming systems with automated requests. While sometimes frustrating for users, CAPTCHAs protect online services including government websites and e-services. Modern CAPTCHAs often work invisibly by analyzing user behaviour rather than requiring explicit challenges. Accessibility concerns exist for users with visual or cognitive impairments.