HTTP headers are key-value pairs sent by the server in response to a request, or sent by the client in a request. These headers metadata about the HTTP request or response and fill a vital role in security, performance, and content negotiation. Examples include the following.
Content-Type: Specifies the media type of the resource `text/html`, `application/json`.
Cache-Control: Provides caching instructions for the browser and intermediate caches.
Authorization: Contains credentials for authenticating the client with the server.
User-Agent: Identifies the client software making the request.
Set-Cookie: Sends cookies from the server to the client.
Accept: Informs the server about the types of data the client is willing to accept.