[Zope] restrictedTraverse versus unrestrictedTraverse versus simpler notations

[nwingfield at che-llp.com]

Q1.  What is the difference between restrictedTraverse and
unrestrictedTraverse?  When would one use the latter over the former?
Q2.  In product code, what is the value in using
'self.restrictedTraverse(id)' or 'self.unrestrictedTraverse(id)' versus
simpler notation, such as 'self[id]' or 'getattr(self, id)' ?