https://developer.apple.com/library/content/documentation/General/Conceptual/DevPedia-CocoaCore/DynamicTyping.html#//apple_ref/doc/uid/TP40008195-CH62-SW2

A variable is dynamically typed when the type of the object it points to is not checked at compile time. Objective-C uses the id data type to represent a variable that is an object without specifying what sort of object it is. This is referred to as dynamic typing.

Dynamic typing contrasts with static typing, in which the system explicitly identifies the class to which an object belongs at compile time. Static type checking at compile time may ensure stricter data integrity, but in exchange for that integrity, dynamic typing gives your program much greater flexibility. And through object introspection (for example, asking a dynamically typed, anonymous object what its class is), you can still verify the type of an object at runtime and thus validate its suitability for a particular operation. 

The following example illustrates dynamic typing using a heterogeneous collection of objects:

NSArray *anArray = [NSArray arrayWithObjects:@"A string", [NSDecimalNumber zero], [NSDate date], nil];

NSInteger index;

for (index = 0; index < 3; index++) {

    id anObject = [anArray objectAtIndex:index];

    NSLog(@"Object at index %d is %@", index, [anObject description]);

}

The object pointed to by the variable at runtime must be able to respond to whatever messages you send to it; otherwise, your program throws an exception. The actual implementation of the method invoked is determined using dynamic binding.

The isa Pointer

Every object has an isa instance variable that identifies the object‘s class. The runtime uses this pointer to determine the actual class of the object when it needs to.

Dynamic typing 动态类型