Cross-Origin Resource Sharing (CORS) is a critical web security feature that enables secure communication between web applications on different domains by using HTTP headers to manage and authorize cross-origin requests. It helps mitigate security issues like cross-site scripting and cross-site request forgery (CSRF), allowing servers to specify which domains can access their resources. CORS involves two main types of requests: preflight requests, which check the safety of complex interactions, and simple requests, which are low-risk and don't require such checks. Proper implementation of CORS includes defining specific origins in headers, avoiding wildcards, conducting regular testing, and potentially using proxy servers to handle requests. Best practices for secure CORS implementation emphasize defining specific origins and avoiding null origins to enhance security. Tools like browser development tools and online CORS testers can be utilized to ensure configurations are secure and effective. Understanding and correctly implementing CORS is crucial for web developers to ensure the security and functionality of web applications, enabling them to perform secure cross-origin HTTP requests and integrate third-party APIs seamlessly.