Difference between Nil and nil and Null in Objective-C

Difference between Nil and nil and Null in Objective-C

I am a relatively experienced iOS developer, however one thing has always
bothered me, what is the difference between Nil(Capital) and
nil(lowercase) and NULL (if any) in terms of performance and usage? I am
absolutely sure that there is a difference between them otherwise why
would they be defined as separately in the first place...