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tnsnames.ora

A tnsnames.ora file maps net service names to connect descriptors. The net service name thus becomes a (most likely shorter and more readable) alias for the somewhat cumbersome net service name.
In the following example, the «text» after the equal sign is the connect descriptor while the text before the equal sign (net_service_name) is the net service name. 
net_service_name= 
 (DESCRIPTION= 
   (ADDRESS=(protocol_address_information))
   (CONNECT_DATA= 
     (SERVICE_NAME=service_name)))
For example: 
ORA10 =
  (DESCRIPTION = (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = somehost)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME=ORA10)
    )
  )

extproc_connection_data

extproc_connection_data is a special service name that is used for external procedures.