Here is a quick summary: A strong reference will keep the object it points to from being deallocated. A
weak reference will not. Thus instance variables and properties that are marked as weak are pointing at
objects that might go away. If this happens, that instance variable or property will be set to nil, instead
of continuing to point to where the object used to live.
原文：http://www.cnblogs.com/playerken/p/4076981.html