The ID of an object. This is used, among other reasons, to refer back to the object at a later time in the program
Used especially in web programming where space on the stack is limited, objectids are used to refer to objects at different parts of the program, and possibly even across different user sessions, if some marshalling is implemented (as databases, etc).
The ID of an object can also be used to determine if two objects are distinct instances or are indeed the same object, in which case, any modifications made on one would be reflected in the other as well
