Start a user session.
  
  Syntax:
Connecting from an SQL Prompt:
   CONNECT "username"
   CONNECT "username/password"
   CONNECT "username/password@connect_string"
   CONNECT "username/password@connect_string AS SYSDBA"
   CONNECT / AS SYSDBA
Starting SQL*Plus:
   SQLPLUS  [option] "<username>" [<start>]
   SQLPLUS  [option] "username/password" [<start>]
   SQLPLUS  [option] "username/password@connect_string" [<start>]
   SQLPLUS  [option] "username/password@connect_string AS SYSDBA" [<start>]
   SQLPLUS                     -- Interactive logon
   SQLPLUS /NOLOG [<start>]    -- Don't login to a database
   SQLPLUS -H                  -- Display SQL*Plus Help
   SQLPLUS -V                  -- Display SQL*Plus version
  Options
     "-L"      Attempt log on just once
     "-M <o>"  Use HTML markup options <o>
     "-R <n>"  Use restricted mode <n>
     "-S"      Use silent mode
     /         Login without using TNS (i.e on the server) ORACLE_SID
     /NOLOG    Don't login to a database
  <start>  ::=
      @<filename>[.<ext>] [<parameter> …]
      @<URI>
  CONNECT can be abbreviated as CONN
  To connect as System Operator rather than DBA, replace SYSDBA with SYSOPER in the syntax above.
Examples
SQL> sqlplus "system/manager@ss64.com"
SQL> sqlplus 
SQL> connect "system/manager@ss64.com"
SQL> sqlplus "sys/manager@ss64.com AS SYSDBA"
C:\> Set oracle_sid=ss64
C:\> Set LOCAL=%oracle_sid%
SQL> sqlplus 
SQL> connect "system/manager"
Passing a username/password on the command line will (on many operating systems) make the password visible in the list of active processes. To minimise the chance of any third party seeing this, use a here string as follows:
Windows:
@echo off
Set oracle_sid=ss64
Set LOCAL=%oracle_sid%
(
Echo connect system/manager;
Echo @sql_update.sql;
Echo Exit;
) | Sqlplus /nolog
Bash:
#! /bin/bash
ORACLE_SID=ss64
Sqlplus -s /nolog <<eof 
Connect system/manager;
@sql_update.sql;
Exit;
eof 
“Everything is connected to everything else” - Barry Commoner (Three laws of Ecology)
Related:
 
COMMIT
DISCONNECT
ROLLBACK
Show user - display the username currently connected
ALTER SYSTEM ENABLE RESTRICTED SESSION 
SET TRANSACTION 
Editing SQL scripts in SQL*Plus
Related Views:
DBA_PENDING_TRANSACTIONS
                                           
USER_RESOURCE_LIMITS
V$SESSION
 
V$SESSION_CONNECT_INFO
 
V$SESSION_LONGOPS