The INSTEAD OF triggers execute instead 
of the original data modification. INSTEAD OF trigger are 
allowed for both tables and views.

Instead of triggers are the procedures that execute in 
place of Data Manipulation language (DML)statement on a 
table.

for example, if i have INSTEAD-OF-UPDATE Trigger on 
TableA,and i execute an update statement on that table,the 
code in the INSTEAD-OF-TRIGGER will be executed instead of 
the update statement that I executed.

None of the above answers are correct..

The Main objective of INSTEAD OF TRIGGERS is.. To handling
the complexity of View.,i.e; to manipulating the
non-updateable views(the views that have created by many
tables with help of joins, Group by operators or set operator's)

if u need more .. contact on my mail:

apparao.akki@oracle.com