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in objc.h
typedef struct objc_class *Class;
typedef struct objc_object {
Class isa;
} *id;
To find out on your own, in XCode, right click id -- or any other type -- and select Jump to definition. It's interesting to note the similarities to other C/C++ based object systems; an object pointer -- an id -- points to a struct that starts with a point to shared class information. I many C++ implementations, that would be the virtual function table, as it would be with Microsoft's COM. In Cocoa, the particulars of objc_class aren't revealed to us in the header file.
The id type is generally declared like:
typedef struct objc_object *id;
This is critical for Objective-C++ where the type is part of a mangled function name.
You can take a look in /usr/include/objc/objc.h
You can refer the doc here : http://opensource.apple.com/source/objc4/objc4-437/runtime/objc.h Hope this will do a favor for you