NSObject is the root class of most Objective-C class hierarchies; it has no superclass. From NSObject, other classes inherit a basic interface to the runtime system for the Objective-C language, and its instances obtain their ability to behave as objects.
Although it is not strictly an abstract class, NSObject is virtually one. By itself, an NSObject instance cannot do anything useful beyond being a simple object. To add any attributes and logic specific to the program, you must create one or more classes inheriting from NSObject or from any other class derived from NSObject. NSObject adopts the NSObject protocol. The NSObject protocol allows for multiple root objects. For example, NSProxy, the other root class, does not inherit from NSObject but adopts the NSObject protocol so that it shares a common interface with other Objective-C (objective-c) objects.
NSObject is the name not only of a class but of a protocol. Both are essential to the definition of an object in Cocoa (cocoa). The NSObject protocol specifies the basic programmatic interface required of all root classes in Cocoa. Thus not only the NSObject class adopts the identically named protocol, but the other Cocoa root class, NSProxy, adopts it as well. The NSObject class further specifies the basic programmatic interface for any Cocoa object that is not a proxy object.
The design of Objective-C uses a protocol such as NSObject in the overall definition of Cocoa objects (rather than making the methods of the protocol part of the class interface) to make multiple root classes possible. Each root class shares a common interface, as defined by the protocols they adopt.
More Information : NSObject Class reference
