tnsnames.ora is a file containing definitions of database services, mapping service names to connection details.<br>
E.g. if you do sqlplus fred/bassett@hounds it may take the name hounds and use the mapping info in tnsnames.ora to establish a connection via SQL*Net to the Oracle database instance with SID = HNDZ on server DOGS (or IP address 1.2.3.4 or wherever).<br>
This mapping information may also be supplied by an Oracle names server if one exists.<br>
sqlnet.ora is a control file for SQL*Net that tells it where to look for its mapping information e.g. if a names server exists it may indicate that this should be tried first when establishing a connection, then if the service name is not defined there, the users local tnsnames.ora file should be checked. It may also contain optional diagnostic and other parameters.<br>