Understanding the OOP

   In our first application, you saw that we introduced Classes and Constructors, now this is a part of the Object-Oriented Programming where we program using Objects, that’s makes it easy for us to code and add a better business logic to your application, C# is a OOP compliant language, and supports it very well.
    OOP is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs.
An object-oriented program will usually contain different types of objects, each type corresponding to a particular kind of complex data to be managed or perhaps to a real-world object or concept such as a customer, an order, or a car.
   Fundamental concepts and features:
   Class: A class is a template for an object, a user-defined data type that contains variables, properties of an object. A class defines abstract characteristics of a thing (object), including its characteristics (its attributes, fields or properties) and the things it can do (behaviors, methods, operations or features). One might say that a class is a blueprint or factory that describes the nature of something. For example, the class Dog would consist of traits shared by all dogs, such as breed and fur color (characteristics), and the ability to bark and sit (behaviors). Classes provide modularity and structure in an object-oriented computer program. A class should typically be recognizable to a non-programmer familiar with the problem domain, meaning that the characteristics of the class should make sense in context. Also, the code for a class should be relatively self-contained (generally using encapsulation). Collectively, the properties and methods defined by a class are called members.

   Instance: One can have an instance of a class; the instance is the actual object created at run-time. In programmer vernacular, the Lassie object is an instance of the Dog class. The set of values of the attributes of a particular object is called its state. The object consists of state and the behavior that's defined in the object's classes.

   Method: A method is a set of procedural statements for achieving the desired result. It performs different kinds of operations on different data types. In a programming language, methods (sometimes referred to as "functions") are verbs. Lassie, being a Dog, has the ability to bark. So bark() is one of Lassie's methods. She may have other methods as well, for example sit() or eat() or walk() or save(Timmy). Within the program, using a method usually affects only one particular object; all Dogs can bark, but you need only one particular dog to do the barking.

   Abstraction: Abstraction means grouping the common behaviors. Abstraction refers to the act of representing essential features without including the background details or explanations. Classes use the concept of abstraction and are defined as a list of abstract attributes.
   Encapsulation: Encapsulation conceals the functional details of a class from objects that send messages to it. For example, the Dog class has a bark() method variable, data. The code for the bark() method defines exactly how a bark happens (e.g., by inhale() and then exhale(), at a particular pitch and volume). Timmy, Lassie's friend, however, does not need to know exactly how she barks. Encapsulation is achieved by specifying which classes may use the members of an object. The result is that each object exposes to any class a certain interface — those members accessible to that class. The reason for encapsulation is to prevent clients of an interface from depending on those parts of the implementation that are likely to change in the future, thereby allowing those changes to be made more easily, that is, without changes to clients. For example, an interface can ensure that puppies can only be added to an object of the class Dog by code in that class. Members are often specified as public, protected or private, determining whether they are available to all classes, sub-classes or only the defining class. Some languages go further: Java uses the default access modifier to restrict access also to classes in the same package, C# and VB.NET reserve some members to classes in the same assembly using keywords internal (C#) or Friend (VB.NET). Eiffel and C++ allow one to specify which classes may access any member.
   Inheritance: Inheritance allows the programmer to treat derived class members just like their parent class's members. This type of relationship is called child-Parent or is-a relationship. "Subclasses" are more specialized versions of a class, which inherit attributes and behaviors from their parent classes, and can introduce their own. For example, the class Dog might have sub-classes called Collie, Chihuahua, and GoldenRetriever. In this case, Lassie would be an instance of the Collie subclass. Suppose the Dog class defines a method called bark() and a property called furColor. Each of its sub-classes (Collie, Chihuahua, and GoldenRetriever) will inherit these members, meaning that the programmer only needs to write the code for them once.
Each subclass can alter its inherited traits. For example, the Collie subclass might specify that the default furColor for a collie is brown-and-white. The Chihuahua subclass might specify that the bark() method produces a high pitch by default. Subclasses can also add new members. The Chihuahua subclass could add a method called tremble(). So an individual chihuahua instance would use a high-pitched bark() from the Chihuahua subclass, which in turn inherited the usual bark() from Dog. The chihuahua object would also have the tremble() method, but Lassie would not, because she is a Collie, not a Chihuahua. In fact, inheritance is an "a… is a" relationship between classes, while instantiation is an "is a" relationship between an object and a class: a Collie is a Dog ("a… is a"), but Lassie is a Collie ("is a"). Thus, the object named Lassie has the methods from both classes Collie and Dog.
(Definitions from Wikipedia)