Here are the definitions of some words used in this manual:
This is the host that the mail client connects to. This server is running the pproxy program. It is responsible for authenticating the user, deciding on which backend server the mailbox of the user is located and what its name is. The proxy will then connect to the backend and forward data between client and backend server.
This is any kind of POP3 or storage server that will be accessed by the proxy server.
This is a backend server that has the POPular pserv running. It is only a place to store the messages.