InnoDB on delete, on update

[Gitte und Ingolf]

Hi out there,
 
i´m using InnoDB with foreign key constraints and i´m looking for the
default behavior of ON DELETE and ON UPDATE.
 
Secondly, what does the ON UPDATE NO ACTION option means? Ist the way it is
used in db2, that every child row must have some matching parent row. If
this condition is not met,the Update fails and all changes are rolled back?
Or is the Update done?
 
What is the meaning of ON UPDATE CASCADE and ON UPDATE SET NULL??
 
Thanks for your help