Questions about Rollback - after insert, update, delete ... operations?

greetings,

I remembered I read something in the mailing list about "*rollback*" a
while ago. People mentioned that some operations cannot rollback.
I cannot remember what kinds of perations are not be able to rollback?

For example,

begin
... ...
insert
... ...
delete
... ...
update
... ...

/* If any of the above operation failed, we can rollback all the
above operations? */
rollback
... ...
end

Will all "Insert, delete, update" operations rollback if any of the
operations fails?

Thanks a lot!
Emi