Difference between value type and reference type ?

Many programming languages provide built-in data types such as integers and floating-point numbers. These are copied when they are passed in to arguments i.e. they are passed "By Value". In .NET terms, these are called Value Types".
The RunTime supports two kinds of Value Types:
1 Built-in value types
The .NET Framework defines built-in value types such as System.Int32 and System.Boolean which correspond and are identical to primitive data types used in programming languages.
2 User-defined value types
The language you are using will provide functionality to define your own Value Types. These user defined Types derive from System.ValueType. If you want to define a Type representing a value that is a complex number (two floating-point numbers), you might choose to define it as a value type. Why? Because you can pass the Value Type efficiently "By Value". If the Type you are defining could be more efficiently passed "By Reference", you should define it as a class instead. Variables of Reference Types are referred to as objects. These store references to the actual data.
The following are the Reference Types:
• class
• interface
• delegate
This following are the "built-in" Reference Types:
• object
• string