Depending on which option you select during the installation of the Oracle program files, you will have a seed database created - seed database sometimes known as a test database. The Oracle Universal Installer launches the DBCA (Database Configuration Assistant), which in turn, creates the seed/test database for you. It is a "test" database because you can use it to confirm the installation of the program files.
The DBCA will create the necessary Net8/Oracle Net Service(8i name of these files/9i name change) files for you to connect to the seed database with whatever name you selected during installation. If you don't know the name of the seed/test database, ask someone who knows, or hunt it down in the tnsnames.ora or listener.ora files. You can check background processes (UNIX) or services (Windows) for running or registered instance names. You can run lsnrctl status and look for database names in the output, assuming the listener is running on the server in question.
Test database is basically a database u create for all u testing purpose.Generally when u install oracle software you can make it a "test database" OR "production" database as oracle will not ask u whether u want to create a "test" DATABASE.Its only u to decide whether u want an installation to be a "test" database or "production" database.