转自:http://www.slyar.com/blog/c-null-nul.html
《C专家编程》里面提到了"The One 'l' nul and the Two 'l' null"，网上查了一下，得到了一个更详细的区分。
NULL is a macro defined in several standard headers, 0 is an integer constant, '\0' is a character constant, and nul is the name of the character constant. All of these are not interchangeable:
NULL is to be used for pointers only since it may be defined as ((void *)0), this would cause problems with anything but pointers.
0 can be used anywhere, it is the generic symbol for each type's zero value and the compiler will sort things out.
'\0' should be used only in a character context.
nul is not defined in C or C++, it shouldn't be used unless you define it yourself in a suitable manner, like:
#define nul '\0'
翻译一下就是：
NULL被大量定义在标准头文件中，0是一个整型常量，'\0'是一个字符常量，而NUL是一个字符常量的名字。这几个术语都不可互换。
1、NULL用于表示什么也不指向，也就是空指针((void *)0)
2、0可以被用于任何地方，它是表示各种类型零值的符号并且编译器会挑出它
3、'\0'应该只被用于结束字符串
4、NUL没有被定义于C和C++，它不应该被使用除非你自己定义它，像：#define nul '\0'