All implementations of WebDriver that communicate with the browser, or a RemoteWebDriver server use a common wire protocol. This wire protocol defines a RESTful web service using JSON over HTTP.
So each WebDriver command is mapped to an HTTP method via the WebDriver service, and then passed on to the HTTP Command Processor to communicate with the browser.
The Command responses are returned as HTTP/1.1 response messages via the WebDriver service.
Different drivers, such as the Firefox Driver and the IE Driver, have different implementations to accomplish the above.
The Selenium WebDriver architecture document linked below goes into further details on how these are implemented and how WebDrvier commands flow through to the browser and back.
Read section 16.6 for details on the Firefox Driver.
The Architecture of Open Source Applications - Selenium WebDriver
by Simon Stewart (creator of WebDriver, and core contributor to the Selenium project)
Also, details on the The WebDriver Wire Protocol will be helpful in understanding how the HTTP methods are mapped.
