主要就以下幾點：
1、nil：一般賦值給空對象；
2、NULL：一般賦值給nil之外的其他空值。如SEL等；
舉個栗子（好重啊~）：
[NSApp beginSheet:sheet
modalForWindow:mainWindow
modalDelegate:nil //pointing to an object
didEndSelector:NULL //pointing to a non object/class
contextInfo:NULL]; //pointing to a non object/class
3、NSNULL：NSNull只有一個方法：+ (NSNull *) null;
[NSNull null]用來在NSArray和NSDictionary中加入非nil（表示列表結束）的空值.
4、當向nil發送消息時，返回NO，不會有異常，程序將繼續執行下去；
而向NSNull的對象發送消息時會收到異常。
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以下轉載自：http://blog.sina.com.cn/s/blog_722017670100v2b4.html
nil: A null pointer to an Objective-C object. ( #define nil ((id)0) )
nil 是一個對象值。
Nil: A null pointer to an Objective-C class.
NULL: A null pointer to anything else. ( #define NULL ((void *)0) )
NULL是一個通用指針（泛型指針）。
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.
[NSNull null]是一個對象，他用在不能使用nil的場合。
因為在NSArray和NSDictionary中nil中有特殊的含義（表示列表結束），所以不能在集合中放入nil值。如要確實需要存儲一個表示“什么都沒有”的值，可以使用NSNull類。NSNull只有一個方法：
+ (NSNull *) null;
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.
example 1:
[NSApp beginSheet:sheet
modalForWindow:mainWindow
modalDelegate:nil //pointing to an object
didEndSelector:NULL //pointing to a non object/class
contextInfo:NULL]; //pointing to a non object/class
example 2: The values1 will only have obj1,
NSObject *obj1 = [NSObject new];
NSObject *obj2 = nil;
NSObject *obj3 = [NSObject new];
NSArray *values1 = [NSArray arrayWithObjects:obj1,obj2,obj3,nil];
example 3: The values2 have 3 objects and the second object is instance of NUSNull
NSObject *obj1 = [NSObject new];
NSObject *obj2 = [NSNull null];
NSObject *obj3 = [NSObject new];
NSArray *values2 = [NSArray arrayWithObjects:obj1,obj2,obj3,nil];
about testing:
if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message] will throw a NSException
testing NSNull: (NSNull *)obj == [NSNull null]