What are the differences between nil, NULL and [NSNULL nil]?

Question

I got an error like this -

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: -[NSCFDictionary initWithObjects:forKeys:count:]: attempt to insert nil value at objects[0] (key: searched)'

Then I tried the below code

if (Obj == (id)[NSNull null])  //Obj is an id 
{
    NSLog(@" Obj is (id)[NSNull null]");    
}
else if (Obj == nil)
{
    NSLog(@"Obj is nil");   
} 
else if (Obj == NULL)
{
    NSLog(@"Obj is NULL");  
} 
else
{
    NSLog(@"Obj is something else");
}

searchedDict = [NSDictionary dictionaryWithObject:Obj forKey:@"searched"];

And I got the NSLog message as 'Obj is nil'.

But when I used the same above code when Obj was NSString and my app got terminated due to nil value, I got the NSLog message as 'Obj is (id)[NSNull null]'.

What are the differences between nil, NULL and [NSNULL nil] ?

Are there different kinds of Null values? if yes, How to check commonly for these null values?

Thanks :-)