Compact, universal identifier for a managed object.
From Apple doc class reference
An NSManagedObjectID object is a compact, universal identifier for a managed object (NSManagedObject). This forms the basis for uniquing in the Core Data Framework. A managed object ID uniquely identifies the same managed object both between managed object contexts in a single application, and in multiple applications (as in distributed systems). Identifiers contain the information needed to exactly describe an object in a persistent store (like the primary key in the database), although the detailed information is not exposed. The framework completely encapsulates the “external” information and presents a clean object oriented interface.
