Hya,
The listener process is responsible to handle all connect request from client. The listener can work in 2 differen modes:
dedicated - for each connect request, the listener start a server process and hands it to the client. this server process will serve the client until it disconnects.
shared (MTS) - there are several dispathcers in the system. each dispatcher works with a server process. when a listener gets the connect request, it asks the most available dispatcher for a server process, the dispatcher is handed to the client and the client works with the dispatcher until it disconnects. In this mode, there are fewer server processes in the system.

The listener.ora file contains information about the listener, its addresss and protocol and the services it serves (the instances). The listener process uses this file for configuration.

The tnsnames.ora file is used to resolve the connect string in the connect request ("connect scott/tiger@orcl") to a database.

Cheers,
R.