Difference between nil vs Nil

They are all zeros, the difference lies in their types

-> The NSNull class defines a singleton object used to represent null values in collection objects (which don’t allow nil values).
 NSNull is a class for objects that represent null.

-> nil is an id. So its points to a non-existent objective-c object

-> Nil is a non-existent objective-c class

-> [NSNull null] is an object that’s meant to stand in for nil in situations where nil isn’t allowed. For example, you can’t have a nil value in an NSArray. So if you need to represent a “nil”, you can use [NSNull null]
Also there is no ‘null’ in the objective c its ‘NULL’ not ‘null’. ‘null’ exist in Java or in C# not in Objective-C

nil -> Null-pointer to objective- c object
NIL -> Null-pointer to objective- c class
null-> null value for C pointer.
NSNUll -> Singleton object used to represent null.

NSNull     [NSNull null]     singleton object used to represent null

nil is the literal null value for Objective-C objects, corresponding to the abstract type id or any Objective-C type declared via @interface. For instance:
NSString *someString = nil;
NSURL *someURL = nil;
id someObject = nil;

if (anotherObject == nil) // do something

Nil is the literal null value for Objective-C classes, corresponding to the type Class. Since most code doesn’t need variables to reference classes, its use is not common. One example is:

 Class someClass = Nil;
 Class anotherClass = [NSString class];