The .NET Framework comes with many different types defined within it.  A type is a class, structure, or other declaration; for instance, int, short, string, DataTable, List<string> are types, with the last one being a generic type.  The following code creates an instance of a type:

Listing 1

int I = 0;

The variable I is of type int, and is an instance of the int type with the value zero.  There are two kinds of types; value types and reference types.  Reference types are defined as a class.  For instance, the following is a reference type:

Listing 2

public class MyClass 
{ 
}

MyClass is a reference type, which means it's stored on the heap, and that it can be explicitly set to null.  Some of the reference types defined in the framework are: string, DataTable, DataSet, List<T>, and so on.  On the other hand, value types are defined as structures, but the value types int, short, etc. inherit from the base class ValueType.  The following is a value type:

Listing 3

public struct MyValueType 
{ 
}

Structures have certain rules, like they need to have at least one field defined within it, as well as a few other rules.  Some of the common value types are: DateTime, Rectangle, int, short, long, and many more.

There are many articles on the web discussing the differences between value and reference types.  Please consult them for more information.