Difference Between Object And NSObject
You should ignore Object.
Objective-C allows multiple root classes. Object is a root class that predates NSObject. It's never something you would want to go use. It's only relevant when something somehow already interacts with Object, so you have to deal with it. This is very rare.
Object doesn't implement -retain and -release, for example.
Objective-C is just the language.
The Cocoa frameworks use the NSObject base class as the root class for the hierarchy. Other implementations use their own root classes, in your case the Object class.
NSObject contains all the infrastructure of the Cocoa framework. In other words it conforms to several protocols that Object does not and will respond to certain methods that Object will not. Specifically see NSObject Class Reference and