A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a security measure designed to determine whether a user is human or automated software. By presenting challenges like word recognition or pattern identification, CAPTCHAs help prevent bots from executing malicious activities such as spamming or data scraping. Implementing CAPTCHAs improves web security by ensuring that interactions on a website are generated by real users. This helps protect sensitive information and maintain the integrity of online services, providing a more secure browsing experience.