I found this activity diagram, in this page, strolling around the internet and i found it very interesting. It shows what happens from the momment the server gets the request from the client and how he decides what response code to send to the client. You must have seen the 404 page not found or the 403 forbidden. Well, according to the RFC there are much much more, as i also posted on the masochists series. But here is a very nice diagram on how things work. Take a peak, it is worth it.

HTTP headers