Object is the superclass of all (non-root) classes. It implements operations common to all types of objects. If you create a new root class, you should reimplement many of the selectors implemented here. However, the only selector that is absolutely required to be implemented is #doesNotUnderstand: (this requirement is enforced by the compiler).
Note that many messages here, such as #==, are implemented only for the benefit of #perform:, since they are inlined by the compiler.