Restricting and cascading deletes are the two most common options.
RESTRICT prevents deletion of a referenced row.
NO ACTION means that if any referencing rows still exist when the constraint is checked, an error is raised;
this is the default behavior if you do not specify anything.
The essential difference between these two choices is that NO ACTION allows the check to be deferred until later in the transaction, whereas RESTRICT does not.
CASCADE specifies that when a referenced row is deleted, row(s) referencing it should be automatically deleted as well.