Oraenv is used to set your ORALCE_HOME and ORACLE_SID.
It is typically used if you are running different oracle products, for example, sqlplus, sqlldr and also forms and reports. Then you use oraenv to change the oracle home to 8.1.7 for sqlldr and dev2k for forms.
You can run it from your .profile or from command line unix. It will prompt you with default oracle home and default oracle sid and you type the new sid and home if change is needed.