Should we consider "sqlplus" as an Oracle client? I am a newbie of SQL*Plus and Oracle, and I have 2 questions about them.  

(1) Executing "sqlplus / as sysdba" command:  

$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Thu Feb 18 05:09:38 2016

Copyright (c) 1982, 2014, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL&gt;


Does the sqlplus connect an Oracle Instance now?  

(2) When use sqlplus connect the local Oracle Instance, like this:  

$ sqlplus username/password


Should we also consider sqlplus as a client? If it is true, does the sqlplus also read tnsnames.ora file? And the requests also forward to listener process?
SQL*Plus is a client that is already available for a very long time. If you use sqlplus username/password then it does not use the tnsnames.ora. It connects to the local database that is defined in the ORACLE_SID environment variable. If you use sqlplus username/password@SID then it will search in the tnsnames.ora file to find out the location of the SID database.