nil,null,[NSNull null]

nil: A null pointer to an Objective-C object.
( #define nil ((id)0)  )

Nil: A null pointer to an Objective-C class.

NULL: A null pointer to anything else. 
( #define NULL ((void *)0)  )

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;