The .NET Framework provides a run-time environment called the common language run-time, which runs the code and provides services that make the development process easier.

Value type: The content of a value type variable or constant is simply a value. For example, the content of the built-in value type, int, is 32 bits of data. Its contents in memory is allocated on the stack. We can define a custom value type with the 'struct' keyword.
public struct Point { public int X, Y; }
Reference type: A reference type has two parts: an  object and the reference to that object. The content of a reference-type variable or constant is a reference to an object that contains the value. A reference type, such as an instance of a class or an array, is allocated in a different area of memory called the heap. We use 'class' keyword to define it.
public class Point { public int X, Y; }

Method overload defines use different type of parameters with  same method name or different set of parameters is known as Method Overloading.
void Foo (int x);
void Foo (double x);
void Foo (int x, float y);

Abstract Class: An abstract class is to provide a common definition of a base class that multiple derived classes can share. It cannot be instantiated.Abstract Member : Abstract classes are able to define abstract members. Abstract members are like virtual  members,  except  they  don’t  provide  a  default  implementation

An enum (also named an enumeration)  is  a special value type that lets you specify a group of named numeric constants. 
Example:
public enum BorderSide { Left, Right, Top, Bottom }