Oracle: Listener.ora / tnsnames.ora / Sqlnet.ora

TNSNames.ora
This file sits on client side. It provides an alias for the client to refer the database.This file will mention the protocol (TCP), Host (database server name) and the port number (default 1521)

SQLNet.ora
This file also sits on the client side. This file contains client side network configuration
parameters. You can specify
1) client domain to append to unqualified names
2) prioritize naming methods
3) Enable logging & Tracing

Listener.Ora
From 9i this file is optional. By default Oracle uses "LISTENER" to listen to the default port 1521. Usually contains
1) Listener name
2) Protocol address that it is acceptiong connection requests
3) Service its listening (SID_LIST_LISTENER).This is optional from 9i