Well, I don't know if I can enumerate all the differences between triggers and stored procedure, but a few:
Triggers are tied to a table as stored procedures are not. Triggers will be activated automatically by DB2 when this table is modified. For each action - delete, insert and update - a trigger has to be created.
Stored procedures are called using the SQL-CALL-statement and a trigger can call a stored procedure.
A stored procedure is nothing more than a subroutine, written in COBOL, C, PL1 or what else, which runs in a seperate address space, either in a db2 controlled or in a WLM managed one.
I hope this gives you an impression what triggers are and what SPs are. They are totally different objects.