What is the difference between BY ACCESS and BY SESSION in Oracle Auditing?

The difference between BY ACCESS and BY SESSION lies in how the DBA_AUDIT_TRAIL data dictionary view records the actions that they capture:

BY ACCESS: Inserts one record in the audit trail for each audited statement.

BY SESSION: Inserts only one audit record in the audit trail, for each user and schema object, during a session that includes an audited action.

Example:

First, enable auditing on the table TABLE1:

AUDIT SELECT, UPDATE,DELETE,INSERT ON HR.TABLE1 BY SESSION WHENEVER SUCCESSFUL;

Secondly, assuming there is activity against TABLE1, check the audit trail:

SELECT  timestamp, os_username, obj_name,userhost, username,action,action_name, ses_actions
FROM    dba_audit_trail
WHERE   obj_name IN ('TABLE1')
AND     username = 'HR'
ORDER BY timestamp DESC;

The query in my case returned the following:

TIMESTAMP	OS_USERNAME	OBJ_NAME	USERHOST	USERNAME	ACTION	ACTION_NAME	SES_ACTIONS
04.07.2011 10:35:20	joe	TABLE1	laptop-joe	HR	103	SESSION REC	---------S------
08.02.2011 18:11:10	joe	TABLE1	appserver1	HR	2	INSERT
08.02.2011 18:11:10	joe	TABLE1	appserver1	HR	6	UPDATE
08.02.2011 18:11:10	jack	TABLE1	appserver1	HR	7	DELETE
08.02.2011 14:50:41	tina	TABLE1	workstation-tina	HR	103	SESSION REC	---S--S---S-----

For statements which are audited BY SESSION, the column SES_ACTIONS will indicate a pattern to view which operation we are talking about.
The 16 characters string in the SES_ACTION column represents different actions, one for each action type in the order (note that positions 14, 15, and 16 are reserved for future use):

* ALTER, AUDIT, COMMENT, DELETE, GRANT, INDEX, INSERT, LOCK, RENAME, SELECT, UPDATE, REFERENCES, and EXECUTE.

The characters are:

* - - None
* S - Success
* F - Failure
* B - Both

Finally, interpret the audit trail:
The first row in the table indicates that a successfull SELECT (the S is found on position 10) was executed on TABLE1, 04.07.2011 10:35:20.
The last row indicates a successful DELETE, INSERT and UPDATE (the S is found in position 4, 7 and 11) in TABLE1 during the session that was audited 08.02.2011 14:50:41.
The three rows where ACTION_NAME is INSERT, UPDATE and DELETE are self-explanatory.

At the tail of the AUDIT clause comes the directive WHENEVER [NOT] SUCCESSFUL which means that oracle should audit only SQL statements and operations that succeed, fail or result in errors. If you omit this clause, then Oracle Database performs the audit regardless of success or failure.