VS 2008 - Values And Reference Types

Reading a book, the following examples are given regarding value and reference types. This example is noted as a value example. ptY display {x = 10, y = 20}

Dim ptX As New System.Drawing.Point(10, 20)
Dim ptY As New System.Drawing.Point
ptY = ptX
ptX.X = 200
Console.WriteLine(ptY.ToString())

This example is noted as a reference example. objY displays "Hello Test".
Dim objX As New System.Text.StringBuilder("Hello World")
Dim objY As System.Text.StringBuilder
objY = objX
objX.Replace("World", "Test")
Console.WriteLine(objY.ToString())

The idea is simple enough, but I just can not tell the difference between the two. Could someone explain how the heap, the stack and the New keyword play a role in accessing directly and indirectly objects and variables?