官方介绍是这样的:

Every object is connected to the run-time system through itsisainstance variable,inherited fromthe NSObject class.isaidentifiesthe object's class;it points to a structure that's compiled from the class definition. Throughisa, an object can find whatever information it needs at run timesuch as its place in the inheritance hierarchy, the size and structure of its instance variables, and the location of the method implementations it can perform in response to messages.

Every object has anisainstance variable that identifies the object's class. The runtime uses this pointer to determine the actual class of the object when it needs to.