LocalTimestamp value difference

Can someone explain me the difference in LOCALTIMESTAMP value when time_zone is set to DBTIMEZONE and LOCAL respectively.

alter session set time_zone =DBTIMEZONE;

SELECT systimestamp,DBTIMEZONE,localtimestamp FROM DUAL;

##############################################

 SYSTIMESTAMP                        DBTIME LOCALTIMESTAMP                

----------------------------------- ------ -------------------------------

28-FEB-13 08.14.03.445364 AM -05:00 -04:00 28-FEB-13 09.14.03.445374 AM

 ####################################################################################

 alter session set time_zone =LOCAL;

 ##############################################

 SELECT systimestamp,DBTIMEZONE,localtimestamp FROM DUAL;

 SYSTIMESTAMP                        DBTIME LOCALTIMESTAMP                

----------------------------------- ------ -------------------------------

28-FEB-13 08.14.05.314946 AM -05:00 -04:00 28-FEB-13 08.14.05.314957 AM