Very recently all the new classes I create in Xcode are prefaced by this extra interface in the .m file:
@interface NewViewController ()
@end
What does Apple expect me to put here and how is it different from the @interface declaration in my .h file?
That is a class extension. In a nutshell, you declare your class’s public interface in the header and use a class extension to declare any private interface you might want to provide (e.g. properties for which you want accessors but which you don’t want to make public).