nil: A null pointer to an Objective-C object.（nil是指的OC空指針）
( #define nil ((id)0) )
Nil: A null pointer to an Objective-C class.（Nil是指的OC類空指針）
NULL: A null pointer to anything else, is for C-style memory pointers.
( #define NULL ((void *)0) )（NULL主要是指的C語言空指針）
NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).
[NSNull null]: The singleton instance of NSNull.
Technically they're all the same,,, but in practice they give someone reading your code some hints about what's going on; just like naming classes with a capital letter and instances with lowercase is recommended, but not required.
If someone sees you passing NULL, they know the receiver expects a C pointer. If they see nil, they know the receiver is expecting an object. If they see Nil, they know the receiver is expecting a class. Readability.
if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message will throw a NSException
