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