A simple name for a service that resolves to a connect descriptor. Users initiate a connect request by passing a user name and password along with a net service name in a connect string for the service to which they wish to connect:
CONNECT username/password@net_service_name
Depending on your needs, net service names can be stored in a variety of places, including:
Local configuration file, tnsnames.ora file, on each client
External naming service, including Network Information Service (NIS) and Cell Directory Service (CDS)