[Zope] restrictedTraverse versus unrestrictedTraverse versus simpler notations

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)' ?