Oracle Network Administration - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 200

Figure 8-1: Understanding the Oracle Network Architecture. Oracle
Net has to be there in both clients and server. Configuration of Oracle
Net on server side is actually the development of listener.ora file.
Configuration of Oracle Net on the client side is actually the
development of tnsnames.ora file. Oracle Net takes care of all different
types of Network protocols. Server and clients know how to speak with
Oracle Net and Oracle Net knows all different Network languages.
tnsnames.ora file contains the resolution of “Net Service Name” or
“Connect String” that we will use to connect to Oracle Server.