Session identifier

Which is the number that identifies uniquely an Oracle Session?
How can an user know it?

I know the session id SID and SERIAL# on table V$SESSION.
This two attributes are necessary to KILL a session.
How can an user know his sesion id SID and SERIAL#?

Besides, Auditing session id exists in the same table too (AUDSID column).
An user can know his AUDSID with: SELECT USERENV('SESSIONID') FROM DUAL

What's the different between AUDSID and (SID,SERIAL#)?

Thanks!