What are object-oriented concepts?

Class: The formal definition of an object. The class acts as the template from which an instance of an object is created at run time. The class defines the properties of the object and the methods used to control the object’s behaviour.
Object: An object is an instance of a class.
Encapsulation: hides detailed internal specification of an object, and publishes only its external interfaces. Thus, users of an object only need to adhere to these interfaces. By encapsulation, the internal data and methods of an object can be changed without changing the way of how to use the object.
Inheritance: A class that derives from another class - known as the base class - inherits the same methods and properties. This promotes reuse and maintainability.
Abstraction: the describing of objects by defining their unique and relevant characteristics (properties). Whilst an object may have 100s of properties normally only those properties of importance to the situation are described. (eg life policies premiums are normally important; whereas the time of day a policy was purchased is not usually of value).
Polymorphism: Allows objects to be represented in multiple forms. Even though classes are derived or inherited from the same parent class, each derived class will have its own behavior. (Overriding and hiding methods)