The TNSNAMES.ORA file describes to Net8 how to communicate with your database. It is commonly referred to as an "alias" file. Basically, you define a database and the user can refer to it using the alias. For example, If I had the following entry in my TNSNAMES.ORA file:
DEV815NT.US =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = mydbserver)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = dev815nt.us)
)
)

I could access my database by:
sqlplus [email protected]