A request header is an HTTP header that can be used in an HTTP request, and that doesn't relate to the content of the message. Request headers, like Accept, Accept-*, or If-* allow to perform conditional requests; others like Cookie, User-Agent or Referer precise the context so that the server can tailor the answer.
Not all headers appearing in a request are request headers. For example, the Content-Length appearing in a POST request is actually an entity header referring to the size of the body of the request message. However, these entity headers are often called request headers in such a context.
In addition, CORS defines a subset of request headers as simple headers, request headers that are always considered authorized and are not explicitly listed in responses to preflight requests.
A few request headers after a GET request:
GET /home.html HTTP/1.1 Host: developer.mozilla.org User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate, br Referer: https://developer.mozilla.org/testpage.html Connection: keep-alive Upgrade-Insecure-Requests: 1 If-Modified-Since: Mon, 18 Jul 2016 02:36:04 GMT If-None-Match: "c561c68d0ba92bbeb8b0fff2a9199f722e3a621a" Cache-Control: max-age=0
Strictly speaking, the Content-Length header in this example is not a request header like the others, but an entity header:
POST /myform.html HTTP/1.1 Host: developer.mozilla.org User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0 Content-Length: 128