In the following explanation, we will use www.example.com as the main server users access when they want to get some kind of service and backend.example.com as the machine that does the heavy work. The main and backend servers are different; they may or may not coexist on the same machine.
We'll use the mod_proxy module built into the main server to handle requests to www.example.com. For the sake of this discussion it doesn't matter what functionality is built into the backend.example.com server—obviously it'll be mod_perl for most of us, but this technique can be successfully applied to other web programming languages (PHP, Java, etc.).