ORACLE_BASE, ORACLE_HOME and ORACLE_SID
 *********************************************************************************
ORACLE BASE

ORACLE_BASE is an environment variable used as base directory for an Optimal Flexible Architecture installation.

On Unix systems, this is usually
/app/oracle or /u01/app/oracle

How to set the environment on Windows:
set ORACLE_BASE=C:\oracle

How to set the environment on Unix/ Linux:
export ORACLE_BASE=/app/oracle
 *********************************************************************************
ORACLE HOME

ORACLE_HOME refers to either:
a directory where the Oracle software is installed
or
an environment variable pointing to the directory where the Oracle software is installed.

Windows
How to check current value on Windows:
echo The current ORACLE_HOME is %ORACLE_HOME%

How to set the ORACLE_HOME environment variable:
set ORACLE_HOME=C:\oracle\ora11.2

On Windows, the ORACLE_HOME may also be stored in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\HOMEID
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\ALL_HOMES
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services

Unix & Linux
How to check current value on Unix/ Linux:
env | grep ORACLE_HOME

How to set the ORACLE_HOME environment variable (valid for bash and ksh):
export ORACLE_HOME=/u01/app/oracle/product/11.2.0/db_1
 *********************************************************************************
Click on the below link for more details on Oracle SID:

ORACLE SID