A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security mechanism used to protect websites from automated bots that can abuse services or extract sensitive information. By presenting users with challenges that require human-like input, such as identifying distorted text or selecting images based on descriptions, CAPTCHAs help verify that a user is indeed human. This protects against spamming, fraud, and unauthorized access, enhancing overall web security and safeguarding user data.