Client PCs access Oracle databases over the network using Net.
Client software, whether written by Oracle or by you, interfaces to Net.
The Net interface is the same regardless of the underlying network transport protocol being used.
The Oracle Net TCP/IP Adapter takes care of transmitting Oracle Net requests over the network using TCP/IP. Other adapters exist as well.
The SPX adapter, for example, is often used in Novell NetWare environments.
The Oracle Net Listener runs on the database server. Its job is to monitor the network for incoming Oracle Net requests.
The Oracle Net Listener passes incoming requests to the appropriate server process.
The server process interacts directly with the Oracle database.
The database server is the computer on which the database resides.