There are two main component of any RDBMS system, one is server process where the actual Database exists, and other is Client process which can connect to server for manipulation of data on server.
The server process in RDBMS system is single program which serves the requests from multiple nodes or clients.
This server process basically installed and configured on server class machine.
The client is individual process that request for connect to database installed on server and can perform activities like insert, update, delete , retrieval or other data manipulation.
All the individual processes that connects to the database and perform some activities
They connect to the server for insert/update/delete /retrieval of data
Some Examples of client processes are:
Connection to the database from the Application
Connection to the database from development machines
Connections made by DBAs for admin activities
How to Connect to RDBMS server
Connection from Client to Server
Oracle database comes with tnsnames.ora, which you can copy to client machine and connect to Oracle database, You need to put the Oracle server information like server information, port, username and password inside tnsnames.ora file, and you can connect throw client like Toad for Oracle or other Oracle client from your client machine.
Connection From an application
In real world from any type of applications like desktop or web, we have requirement to connect the database and do some operations like storing or retrieving the data.
So in this case we need to connect the database server from an application, you can prefer any language for developing application, but the way of connecting to database are similar for any type of language.
You can connect from application which has been developed in any language can be connect by Open Database connectivity(ODBC).
Java application can be connected by ODBC and Java also provide JDBC API to connect to any RDBMS database.
Connection through a utility
There are some utility software used for connecting to Oracle database, the example of these client software are Sqlplus, SQL loader, Tod for Oracle.