Identity : Every instance of a class should be unique. e.g.
in memory, every object has its own memory address. This is
part of the unique id, other than which a user can have
member variables uniquely identifying the instance of the class.

State : Every object, at any given point of time would have
to have a set of attributes defining its State.

Behavior : Every object based on its state and optionally
identity will have particular behavior.

Object is an instance of the class.And oject must have the 
identity because of object can be specified in Class. And 
has carries the behaviour of that perticular class