After the HTTP client sends HTTP Request, the Web server sends HTTP response back. It consists of HTTP headers (including status line) and with optional message body separated by a blank line. All the Web content received from server is included into the message body.
Headers passed not only indicate the type of content and suggested handling of it, they also can hint to used the cached version of the content, if applicable. Headers can indicate content length, encoding and other parameters required to parse the response and perform whatever actions are expected (i.e.. display in a browser window). While checking the Web resource parameters, both headers and message can be used.