Simple definition
Cross-Origin Resource Sharing uses HTTP response headers to permit selected browser requests across different origins.
Purpose and use
CORS, Cross-Origin Resource Sharing, lets a server state when a browser may expose a response to code from another origin.
How does it work?
The browser interprets Access-Control-* fields and may send an OPTIONS preflight. Credentialed requests have stricter rules than public requests without credentials.
Example and verification
For a blocked call, compare the exact origin, permitted methods and headers, and the request's credentials policy.
Points to watch
CORS does not prevent every request from being sent and replaces neither authentication nor CSRF protection. Non-browser clients need not enforce it.
In short
CORS mainly governs browser code's access to cross-origin responses; server security must remain independently enforced.