What's the difference between a simple CORS request and a preflighted CORS request?

A 'simple' CORS request is a GET, HEAD, or POST request that doesn't use custom headers or certain non-standard content types. These requests are sent directly to the server. A 'preflighted' CORS request is any request that is not simple, such as requests using PUT, DELETE, PATCH, or custom headers, or requests with specific `Content-Type` headers like `application/json`. Before making a preflighted request, the browser first sends an HTTP OPTIONS request (a 'preflight' request) to the server to check if the actual request is allowed. The server's response to the OPTIONS request determines if the actual request can proceed.