They differ in their types. They're all zero, but NULL is a void *, nil is an id, and Nil is a Class pointer.

各种类型的不存在的表示。

理解“不存在”的概念不仅仅是一个哲学的问题，也是一个实际的问题。我们是 有形 宇宙的居民，而原因在于逻辑宇宙的存在不确定性。作为一个逻辑系统的物理体现，电脑面临一个棘手的问题，就是如何用 存在 表达 不存在 .

在 Objective－C 中，有几个不同种类的 不存在。

They both are just typecast zero's. Functionally, there's no difference between them. ie.,

#define NULL ((void*)0)
#define nil ((id)0)

There is a difference, but only to yourself and other humans that read the code, the compiler doesn't care.

One more thing nil is an object value while NULL is a generic pointer value.

https://stackoverflow.com/questions/1564410/when-should-i-use-nil-and-null-in-objective-c