The set of entities in a class share the same conceptual basis. An object is an instance of a class. Classes serve as templates for the creation of objects. Most object-oriented languages support a type of classes called abstract classes. An abstract is a class C, with at least one operation left unimplemeted.
Objects communicate through message passing.
This occurs when an object receives a message. A message is an action asking objects to behave in some way. The execution of an object- oriented program involves the receiving, interpreting and responding to messages from other objects. There are basically two ways of using inheritance. These are interface inheritance and implementation inheritance.