Trigger is just like a storedprocedure but the only 
difference is it is automatically call when the event ocour
or fire.

Trigger is procedure which is automatically fires when we 
modify the data in table(After i.e.,Data Manipulation wiil 
be one)

A trigger is a DB object like proc. or func. stored in data 
dictonary. It will fire implicitly with event on which it 
written.

Main use of a trigger are

-- we can take backup of data.
-- Can be used for auditing purpose. 
-- For applying complex bussiness rule.
   Etc.

Triggers are simply stored procedures that are executed 
automatically by the database whenever
some event (usually a table update) happens.

sample:

CREATE [OR REPLACE]
TRIGGER trigger_name
BEFORE (or AFTER)
INSERT OR UPDATE [OF COLUMNS] OR DELETE
ON tablename
[FOR EACH ROW [WHEN (condition)]]
BEGIN
DBMS_OUTPUT.PUT_LINE(’Ur message to be 
displayed ’  :NEW.NAME);
END;

Trigger is a special kind of stored procedure executed 
certain event occurs.

trigger is used nto change the control from one state to 
another

trigger is stored procedure it execute the one event
when another event is happenning for example it can do the 
insert operation as well as select operation.