HTTP Architecture

I've now managed to run OpenJMS with the HTTP configuration. However, 
I'm a little confused about the architecture. For what exactly are the 
servlets used? I've got the impression that it works as follows:

client -> server servlet -> openjms server -> client servlet -> client

Is that correct? If the client have the ability to use HTTP directly 
(without being a servlet), why doesn't the server? How do the servlets 
communicate with the server?

Flo