Curl is one of the most widely used tools in the web industry. If you are not familiar with it, I highly suggest you read up on it. It is a CLI (Command Line Interface) tool.
Just below you see curl with the URL we use to toggle LED #13 on the Yśn. The -v option tells curl to use the verbose option and give us the HTTP headers as also.
$ curl -v http://arduino.local/arduino/digital/13/0
In case it is not apparently clear, let explain what is happening. In the dump above from curl, when you see the > and < characters at the beginning of the line you are actually seeing the HTTP protocol.
It is as simple as what you see, the browser sends (>) multiple lines - with each field delimited with a new line break. And each field has label (or name) and a value. It's that simple. The webserver response (<) in a similar fashion.
Details of the actual HTTP protocol syntax can be read in RFC 2616.