In Windows, go to Control Panel->Administrative Tools->Services and look for any service with a name similar to 'OracleServiceSID.' The SID is your instance name. There will be one instance for each database.
In Unix/Linux, look for a file called "oratab" in either the /var/opt/oracle directory or the /etc directory. This file will contain one entry for each instance.
Unfortunately, there is no native method in SQL*Plus to see the above information.
To see the list of tables in the database, you can query the DBA_TABLES Data Dictionary view, similar to the following:
SELECT owner,table_name FROM dba_tables;
The Data Dictionary views are fully described in the Oracle 10g Reference Guide.
This was first published in October 2006