How to connect to another database using sqlplus?

Assume you already logged into an oracle database, now you want to log into another database.
The following are suggestion for you:
- Connect to a local oracle database:
Method: connect username/password
- Connect to a remote oracle database:
Method: connect username/password@net_service_name

[net service name]: A simple name for a service that resovle to a connect descriptor.
Its format likes "SID_OracleHost".
Depends on your needs, net service name can be stored in a variety of places, including:
(1) Local configure file, tnsnames.ora, on each client
(2) Directory server
(3) Oracle Names server
(4) External naming service, such as NIS or CDS
For oracle9i,you can simply connect to the host database using Oracle Enterprise Manager
Console, the net service name is stored in tnsnames.ora aotumatically. And you only can
remove the net service name by modifying tnsnames.ora manually.