The interfaces to objects defined by a library are specified by object type definitions. An object type defines only a set of messages which may be sent to an object over various phases of its lifetime. An object type makes no commitment to the classes that might be used to implement the object.

Multiple classes may all implement the same messages belonging to a type. The independence of types and classes means that different classes can provide alternate implementations of the same object type. For example, a particular implementing class might be selected to optimize the implementation for a particular case.

Object types are similar to protocols defined by Objective C, and the declarations appearing in a library header file are a minor adaptation of Objective C protocol syntax. A key difference from protocols is that object types are published as real external objects that may be used at runtime to create instances of a type. A further difference is that the object types of the defobj library are divided into separate interfaces that define distinct phases of an object's life cycle.