Introduction

The programming in which data is logically represented in the form of a class and physically represented in the form an object is called as object oriented programming (OOP). OOP has the following important features.

Class

In OOP languages it is must to create a class for representing data. Class contains variables for storing data and functions to specify various operations that can be performed on data. Class will not occupy any memory space and hence it is only logical representation of data.
Refer following article for depth explanation 

• Types of Classes in C#

Encapsulation

Within a class variables are used for storing data and functions to specify various operations that can be performed on data. This process of wrapping up of data and functions that operate on data as a single unit is called as data encapsulation.
Refer following article for depth explanation

•  Encapsulation in C#

Abstraction

Within a class if a member is declared as private, then that member can not be accessed from out side the class. I.e. that member is hidden from rest of the program. This process of hiding the details of a class from rest of the program is called as data abstraction. Advantage of data abstraction is security.

Refer following article for depth explanation 

• Abstract class in C# 

Object and Instance

Inheritance

Creating a new class from an existing class is called as inheritance. When a new class requires same members as an existing class, then instead of recreating those members the new class can be created from existing class, which is called as inheritance. Advantage of inheritance is reusability of the code. During inheritance, the class that is inherited is called as base class and the class that does the inheritance is called as derived class.
Refer following article for depth explanation  

• Types of Inheritance in C# 

Polymorphism

Polymorphism means having more than one form. Polymorphism can be achieved with the help of overloading and overriding concepts. Polymorphism is classified into compile time polymorphism and run time polymorphism.
Refer following article for depth explanation

• Polymorphism in C# .

Refer the following articles to learn more about OOPS concept.

• Programming Methodologies
• Types of Classes in C#
• Polymorphism in C#
• Types of Inheritance in C#
•  Abstract class in C#
• Method Overloading and Method overriding C#
•  Sealed Class in C#
• Encapsulation in C#   
• Interfaces in C#  
• Static Class in C# 

Summary

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